ESO Libraries

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v. L. T.SITE SELECTION WORKING GROUP

FINAL REPORT

November 14, 1990

Edited by M. Sarazin

Contents

1 INTn.oDUCTION1.1 Composition of the VI;r Site Selection 'vVorking Group (1988-1990) .1.2 Introdudion .1.3 Terms of Reference . . . . . . . . . . . . . . . . . . . . . .

1.3.1 Terms of reference . . . . . . . . . . . . . . . . . .1.~.2 Three relliarks by the chairman 01' thc VL'1' SSWG

1.4 Activities of the \Vorking Group .

2 THE ESO VLT SITE EVALUATION2.1 Daekgl'ouncl......... ..2.2 Expericnce of La Silla as a Site2.3 A Site for the ESO VLT . . ..2.'1 First Site Survey .2.5 Large-Scale Identification of Site Candidates .2.6 Paranal . . .2.7 Arma.zoni . . . . . . . ..2.8 Other Coastal l\'Iountains2.9 Inland l\'loulltaills .2.10 Fi I'st concl usions .2.11 Sites outside Chile2.12 La Reunion ....

3 METHODSANDINSTRUMENTS3.1 Jnl,l'odllctioll .3.2 Evaluation of Cloud Cover .3.3 l\1easurements of Atmospheric Water Vapour

3.3.1 Introduction .3.3.2 The sky-radiance monitors3.3.3 Ca librations ...

3.4 Light Pollution SOlll'ces3.G Sccillg monitor ...3.6 l\licrothcrmul sensor3.7 Scintillometer..3.8 Acoustic soundcr . .3.9 Dust meter . . ...3.10 Wind f10w modelling

1123344

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2120222323242527283031313233

CONTENTS

4 RESULTS OF THE TEST CAMPAIGN4.1 Loeation and eharaeteristies of sites U1H.ler eonsideration

4.1.1 Geography .4.1.2 Overview of available data .4.1.3 Topographie maps

4.2 Cloud Cover ..4.2.1 La Silla .4.2.2 Paranal .4.2.3 Grand Bellare .

4.3 'Vater Vapour in the Atmosphet'c'1.:3,1 La Silla arca4,3.2 Paranal area .4,3.3 Grand ßCllare .

4.4 I\Iet.eorology......4.4.1 Summary of meteorologieal data4.4.2 Wind .4.'1.3 Relativc llumidity4.'1.4 Temperatmc

4.5 Sceing .4.5.1 Introduetion4.5.2 La Silla arca4.5.3 Parana! area

4.6 Scintillation and upper ntmospheric Willd4.6.1 Introdllction4.6.2 Radiosondc data4.6.3 Scintillation ...

4.7 Ground level turblllenee4.8 Dust .

4.8.1 Laborat.ory t.ests4.8.2 MeaSlll'emellt.s insiele thc tclcscopcs ,,1.8.:1 Paranal arca'1.8.'1 La Silla urea ..... , .... , ..

5 ANALYSIS5,1 Clouelincss........................

5.1.1 Aeeuraey of the ll1easurel1let1l.s ,5.1.2 Cloueliness in the La SiUa anel Parallai arcas

5.2 Preeipitable Water Vapour in the Atmosphcre . , .5.2.1 Aeeuraey of the measurements ..... , ..5.2.2 Preeipitable Water VapoUl' above La Silla and Paranal .

5.3 l\Ieteorology .5.3.1 Relative humic1ity5.3.2 Tcmperatlll'c5.3.3 Winel .

5.4 Seeing .5.4.1 Aecuracy of the measurements5.4.2 Quality of the elata base ....5.4.3 Comparison La Silla-Vizcachas5.4.4 Comparison Pm'anal-Vizcaehas5.4.5 Comparison La l\Iontura-Paranal

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35353537394343555668687475848484929295959696

101101101101105105105107107107

109109109111119119120126126126128130130131132132133

CONTENTS

5.4.6 Comparison Armazoni-Paranal . . . .5.4.7 Comparison Vizcachas-Las Campanas5.4.8 Seasonal variations of seeing .

5.5 Other parameters. . . . . . .5.5.1 Airglow .5.5.2 Thermographie survey5.5.3 Light pollution5.5.4 Seismieity

5.6 Summary . . . ...

6 FIGURES OF MERIT6.1 Introduction .....6.2 Selection of site parameters

6.2.1 Introduetion ....6.2.2 Site requirements for infrared6.2.3 Speekle lifetime . . . .

6.3 Determination of power laws6.3.1 Direet imaging6.3.2 Speetroscopy .6.3.3 Interferometry

6.4 Results .6.5 Figures of exeellence .

7 RECOMMENDATIONS

8 ACKNOWLEDGMENTS

A SEEING IN THE WAKE OF THE NTT nUILDINGSA.1 Introduetion ...A.2 Instrumentation.A.3 l\'lethouology ..AA Results .

AA.1 Sclcction of the wind dircctionAA.2 Determination of tlle relative thermal turbulence in the wakeAA.3 Wake equivalent seeing .

A.5 Conclusion .

B SEISMIC HAZARDB.1 Seismicity in Chile .B.2 Seismicity in the Vizcachas and Paranal areas .

n.2.1 International Seismology Center Data BaseB.2.2 Regional analysis of seismicity in Chile.B.2.3 Loeal Analysis of Seismic lIazard .

B.3 Conclusion .n.3.1 Regional aspect .B.3.2 Local aspect ..

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145145146146147147149149150152155156

159

160

161161162163165165166168170

171171172172172173176176176

List of Figures

2.1 Gcogl'aphiealloeatioll of Paranal

3.1 Sceillg monitor . . . . .3.2 Mcthod of mcasurement3.3 Aeoustie sounder .

4.1 Mnp of Armazoni .4.2 Map of Paranal ..4.3 Map of La Montura4.4 Mnp of Vizeaehas . .4.5 Photometrie nights, La Silla 1965-19671.6 Photometrie nights, La Silla 1968-1970 .4.7 Photometrie nights, La Silla 1971-1972 .1.8 Photometrie nights, La Silla 1965-1972 .1.9 Photometrie nights, La Silla 1983-1985 .1.10 Photometrie nights, La Silla 1986-1988 .1.11 Photometrie nights, La Silla 1989 ....'1.12 Photometrie nights, La Silla 1983-1990 .4.1:3 Photometrie nights, La Silla .4.ltI Speclroseopie night.s, La SiJla 1983-19854.15 Spectroscopie nights, La Silla 1986-19884.16 Speetroseopie nights, La Silla, 19894.17 Speetroseopie Ilights, La Silla, 1983,19904.18 Photometrie nights, 12 months filter, La Silla 1965-19724.19 Photometrie nights, 12 months filter, La Silla 1983-19901.20 Spectroseopie nights, 12 months filter, La Silla 1983-19904.21 Phot.ometrie nights, Paranal 1983-1985 .4.22 Photometrie nights, Paranal 1986-1988 .4.23 Photometrie nights, Paranal 1989 . . . .1.21 Photometrie nights, Paranal 1983-1990 .4.25 Spectroseopie nights, Paranal 1983-19854.2G Speetroseopie nights, Parana11986-19884.27 Spectroseopie nights, Paranal 1989 ...1.28 Speetroscopie nights Paranal, 1983-19904.29 Photometrie nights, 12 months filter, Paranal 1983-19904.30 Speetroseopie nights, 12 months filter, Paranal 1983-19904.31 Water vapour, nighttime, Winter La Silla .....4.3:l Water vapour, nighttime, Spring and Fall La SiUa .4.3:3 'Water vapour, nighttime, Summer La Silla .

iv

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394041424747484849495050515152525353545463636464656566666767707071

LIST OF FIGURES

4.34 Water vapour, daytime, Winter La Silla .....4.35 Water vapour, daytime, Spring and Fall, La Silla4.36 Water vapour, daytime, Fall La Silla ....4.37 Water vapour, nighttime, seasOilal La Silla .4.38 Water vapour, daytime, seasonal La Silla .1.39 Water Vapour, nighttime, Grand ßenare .1.10 WV Monit.ors eomparison (1,2), 20J-L1n ..4.41 WV Monitors eomparison (1, 2), 26.7J-L1n4.12 WV Monitors eomparison (1, 3), 20/.L1714.43 WV Ivlonitor eomparison (1, 3), 26.7J-L1n .4.14 Water vapour, nighttime, Winter Paranal4A5 Water vapour, nighttime, Spring and Fall ParallaI4.46 Water vapour, nighttime, Summer Paranal .4.47 Water vapour, daytime, Winter Paranal .4.18 Water vapour, daytime, Spring and Fall Paranal1.49 'Vater vapour, daytime, Summer ParallaI .4.50 Water vapour, llighttime, seasonal Paranal .4.51 'Vater vapour, daytime, seasollal Paranal1.52 Windrose at La Silla . . . . ..'1.G:3 'Vind speed, La Silla (monthly)4.51 Windrose at Vizeaehas ..4.G5 ,,,rind at Le Grand ßenare .11.56 Windrose at ParallaI .'1.G7 Wind speed, Paranal (monthly)1.58 Windrose at La l\Iontura .4.59 Wind at Armazoni .1.60 Wind speed ArmazonijParanal'1.G1 Relative humidity, La Silla (monthly) .'1.G2 Relative humidity, ParallaI (monthly)1.63 Temperature, La Silla (monthly)4.64 Temperature, Paranal (monthly)'1.65 Seeing statisties, La Silla .4.GG Seeillg at Vizeaehas ..'1.G7 Seeing at Paranal ...1l.(5$ Seeing at La Montura4.G9 Wind speed at 200mß1.70 Wind at 200mß and seintillation1.71 Cross-ealibration of seintillometers1.72 Turbulenee at ground level versus height4.73 Turbulenee at ground level: diurnal eycle .4.74 Dust at La l\Iontura4.75 Dust at Vizeaehas .

5.1 Photometrie nights, monthly Paranal-La Silla5.2 Photometrie nights, monthly differenees Paranal-La Silla5.3 Photometrie nights, monthly, filtered Paranal-La Silla5.4 Photometrie nights differenee, filtered Paranal-La Silla5.5 Photometrie nights, seasonal, Paranal-La Silla5.6 Spcctroseopie nights, monthly Paranal-La Silla5.7 Speetroseopie nights differenee Paranal-La Silla

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717272737375787879798080818182828383848787899090919192939394949898

100100102103103106106108108

112112113113114116116

LIST OF FIGURES

5.8 Spectroscopic nights 12 months filter, Paranal-La Silla .5.9 Spectroscopic nights difference, filtered Paranal-La Silla5.10 Spectroscopic nights, seasonal, Paranal-La Silla .5.11 Water vapour, nighttime, Winter, Paranal-La Silla .5.12 Water vapour, nighttime, Spring and Fall, Paranal-La Silla5.13 Water vapour, nighttime, Summer, Paranal-La Silla .5.14 Water vapour, daytime, Winter, Paranal-La Silla .5.15 Water vapour, daytime, Spring and Fall, Paranal-La Silla5.16 Water vapour, daytime, Fall, Paranal-La Silla ....5.17 Water vapour, nighttime, seasonal , Paranal-La Silla5.18 Water vapour, daytime, seasonal , Paranal-La Silla5.19 Water vapour ,Paranal-Armazoni .5.20 Relative lIumidity Paranal/La Silla .5.21 Relative diurnal temperature cycles .5.22 Relative nocturnal temperature variation5.23 Wind speed statistical distribution5.24 Wind speed diurnal cycle ...5.25 Seeing: cumulativc probability .5.26 Scintillation Vizcachas-Paranal5.27 Secing, Paranal-La Montura ..5.28 Sccing Vizcachas-Las Campanas5.29 Sccing Vizcachas-Las Campanas (bins)5.30 Correlation of seeing data5.31 Scasonal variation ..

6.1 Best seeing at Paranal6.2 Best seeing at Vizcachas

A.1 Location of the seeing monitorsA.2 Comparison of secing records .A.3 Relative differential thermal turbulence versus wind speed: all dataA.4 NTT top view ...A.5 J anuary wind rose .A.6 Fitted polynomials .A.7 Equivalent wake FWIIM

B.1 Schematic profile of the subduction zoneß.2 Seismic sources for Chile .B.3 Gutenberg-Richter LawBA Seismic IIazard Curves ..

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List of Tables

4.1 Cloud cover, La Silla 1965-19724.2 Cloud cover, La Silla 1983-19864.3 Cloud cover, La Silla 1987-19904.4 Cloud cover, Grand Benare 1986-19874.5 Cloud cover, Paranal 1983-1986 .4.6 Cloud cover, Paranal 1987-1990 .4.7 Cloud cover, daytime, Paranal 1983-1986 .4.8 Cloud cover, daytime, Paranal 1987-1990.4.9 Cloud cover, morning and evening, Paranal 1983-19864.10 Cloud cover, morning and evening, Paranal 1987-19904.11 Water vapour, La Silla nighttime4.12 Water vapour, La Silla daytime .4.13 Water vapour, Paranal nighttime4.14 Water vapour, Paranal daytime .4.15 Meteorological characteristics (Paranal area)4.16 Meteorological characteristics (La Silla area)4.17 Meteorological characteristics (La Reunion)4.18 Wind speed, La Silla area4.19 Wind direction at La Silla4.20 Wind speed ,Paranal area4.21 Wind direction at Paranal4.22 Seeing Vizcachas: 1mn avcrages4.23 Seeing Paranal: 1mn averages .4.24 Seeing La Montum: 1mn averages.4.25 Wind speed at 200mB . . . ....

5.1 Cloud cover, measurement accuracy5.2 Seeing Vizcachas-Paranal ...5.3 Scintillation Vizcachas-Paranal5.4 Seeing La Montura-Paranal5.5 Seeing Armazoni-Paranal .5.6 Original data: 1mn averages

6.1 Power relation (direct imaging)6.2 Power relation (spectroscopy) .6.3 Power relation (interferometry)6.4 Relative merit . . . . . . .6.5 Coefficients of excellence . . . .

vii

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LIST OF TABLES

7.1 Summary of medians ..

A.l Summary of seeing data

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Chapter 1

INTRODUCTION

1.1 Composition of the VLT Site Selection Working Group(1988-1990)

• J.-P. Swings, chairman of the WG, University of Liege, Bclgium

• 1. Appcllzeller, Heidelberg ObservaLory, FRG

• A. AreIcbcrg, Lund ObservaLory, Sweden

• P. Charvin t, Paris ObservaLory, France (chairman ESO Scientific anci Tech­nical CornmiLtee)

• G. LeW:~vre, Paris Observatory, France

• C. Perrier, Lyon Observatory, France

• :M. Sarazin, secretary of the WG, ESO Garehing

• H.-E. Schuster, ESO Chile

• P. Shaver, ESO Garching (chairman ESO's VLT Operations Working Group)

tdeceased Jan. 24, 1990

In addition to the above members, the following ESO members were perma­nently invited:H. van eIer Laan (ESO, Dir.Gen.)J. Beckers (ESO-VLT interferometry)D. Enard (ESO-VLT project)M. Tarenghi (ESO-VLT projeet)

1

CI-IAPTER 1. INTRODUCTION

1.2 Introduction

2

To begin this introduction it is appropriate to quote some thoughts expressed inwhat is rcferred to as the "BIue Book", i.e. the "Proposal for the constructionof the 16-m Very Large Telescope" (issued in March 1987). In particular, somecxcerpts from chapter 9 "Site" are relevant and are reproduced here.

"The requirements for astronomical sites have been often described. The idealsite should have 1010 atmospheric turbulence, 1010 water vapor and dust content,a 1010 frequency of clouds, 1010 wind and a small daily (and particularly nightly)tempemture variation. Absence of atmospheric pollution and nearby sources isalso important.

The present ESO site at La Silla is one of the world 's better sites with respeet tothese cl'iteria. Although the atmospheric water vapor is mther high, there are alsopel'iods with less than 2mm precipitable H 20. The site was chosen almost 25 yearsago on the basis of then valid criteria which included, apart from astronomicalconsidemtions, questions like availability of watel', nearness to a reasonably largecity, etc. In view of the magnitude of the VLT investment, it is not cleal' that thelaller criteria should have the same weight today. Consequently, a new searchlws been made to see if substantially bettel' sites than La Silla exist, withoutconside'ring ease of opemtion, infmst1'ueture rcquil'ements, or pel'sonal comf01't.lor the staf]. Only when the facts are known does it become reasonable to makethe tmdeoffs between the astronomical and other aspeets.

(...) FlOm all that is known to-date, there are no suitable sites for the VLTin A ustmlia. While the surveys made by ESO and others in the fifties and six­ties have located some good sites in southern Africa, none appeared to oe fullycompamble to La Silla. The new ESO surveys have therefol'e been concentmtedon various sites in Chile", and to alesseI' extent on the island of La Reunion.Details about these surveys appeal' in chapters 2-4 of the present document.

The principal conclusion to be drawn from surveys condueted since September19S3 (see chapter 2 for a historical review) is that both the cloudiness and theaLmosphcric water vapor content are particularly low in a coastal arca of northernChile, especially in the area near the isolatcd mountain "Cerro Pm'anal". Detailedstudies of sites in that area were thus conducted, as weIl as in the La Silla arca.This rcport will deal with the surveys in these two areas, that will be caIled"Paranal area" and "La Silla area".

As mentioned above, chapter 2 will review the activities that have led to retain­ing the two areas as good candidates. Chapter 3 then presents the instrumentsuscd in the site testing campaigns: these instruments have, for a large part,been defined and recommended by the first VLT site selection working group 1

whose aetivities lasted from 1984 to 1988. Chapter 4 deals with the different site

I The first working grol\P consisted of H. van der Laan (chairman), A. Ardeberg, J. Vernin, G.Wcigclt, H. Wohl (membed), D. Cadet, F. Roddier (consultants), J.-P. Swings (chairman VLT Advi­SOl)' Committee), D. Enard and M. Sarazin (VLT project group)

CIIAPTER 1. INTRODUCTION 3

parameters (astronomical as weH as meteorological), that are then analyzed inchapter 5 and quantified in Chapter 6. Finally, chapter 7 contains the recommen­dations presented by the VLT Site Seleetion Working Group that was formed inDecember 1988 (see its terms of reference in the next paragraph).

1.3 Terms of Reference

1.3.1 Terms of reference

The terms of reference for a VLT Site Selection Working Group (SSWG), asdefined by ESO's Director General in December 1988, are given hereafter :

• "Membe1'ship

About four per'sons, to cover the main areas of visible and infraredobserving as well as interferometr'ic imaging. The ESO Direetor Gen­eral appoints these membc1's and the chairman 0/ the working group inconsultation with the chairman of the STG. The composition may bechanged when pa1't 2 0/ the assignment begins.

The head 0/ site measurements in the VLT Enginee1'ing G1'OUP andthe head of VLT Site Services in Chile. The chairman 0/ the VLTOperations Working Group.

The membe1's 0/ the VLT Management Team will have a steady invi­tat ion to participate in working 91'OUP meetings.

• Assignment

To advise the Director Gener'al on the analysis and interpretation ofVLT site data.

To analyze (1) the ast1'Onomical, (2) the operational and financial prosand cons of the several site options.

To prepare and submit to ESO management a recommendation on thebest possible choice for the VLT site.

• Schedule

The first pa1't 0/ the assignment is to be completed in about 18 months.The second part is to lead to conclusions and recommendations beforethe end of Oetober 1990."

CHAPTER 1. INTRODUCTION 4

1.3.2 Three remarks by the chairman of the VLT SSWG

• In its assignments the present working group has so far only consideredthe scientific characteristics of the sites under consideration, and has thuspurposely not dealt with any matter related to finances, operations, 01'

personnel.

• It should also be made deal', as expressed in this introduetion and in thesubsequent chapter on the history of site surveys, that, following an earlierchoice among potential areas, the VLT SSWG has essentially been asked tomake a seleetion between sites in the "Paranal area" and sites in the "LaSilla area". The recommendation formulated in this document is thus to beunderstood in that context.

• Although the contributions of ESO staff members to the present reporthave been substantial, and are indeed gratefully acknowledged, it shouldbc stressed that the condusion of the analysis, and, a fortiori, the recom­menclations represent the unanimous opinion of the external, i.e. non ESO,SSWG members.

1.4 Activities of the Working Group

The VLT Site Seleetion Working Group met six times in Garching betweenFebruary 1989 and May 1990, and paid a one-week visit to Chile at the endof September 1989 which induded several meetings on La Silla and in Antofa­gasta. The minutes of all meetings are available upon rcquest, from M. Sarazin(ESO, Garching).

Thc SSWG was always presented with a status report on thc site aetivitiesin Chile (induding meteorological as weIl as astronomical data), and with theavailability and reliability of the testing instruments. In order to exemplify someof the activitics of the SSWG (induding requests on specific observations on par­tiCldar sites), we reproduce hereafter the short report delivered by its chairmanat the 24th meeting of the STC on Nov. 27 and 28, 1989:

"The SSWG held several meetings on La SiUa and in Antofagasta, fiew overVizcachas, dose to Las Campanas, over and around Paranal, La Montura andArmazoni. The SSWG visited Vizcaehas, La Montura, and Paranal (+ the "NTThiU"), and had a look at Armazoni from a few kilometers away. The SSWG wasimpressed by the quality of the site testing (under the guidanee of Mare Sarazin),aud by the hospitality of the ESO staff in Chile (the visit organized by H,ans-EmilSehusle1' was both eomfortable and eonstruetive). The panel also appreciated theseleetion of the sites to be eonsidered for the VLT through the previous efforts ofA. A rdeberg and hiseoUaborators.

I

The SS WG was shown the seeing monitors during daytime and at night; evensimultaneous observations were performed on La SiUa with the NTT and a see-

CHAPTER 1. INTRODUCTION 5

ing monitor. The SSWG thus gained faith in the reliability of the seeing valuesproduced by the monitor's.

The SSWG is now in the process of preparing its final report: a considerableamount of time was devoted to the organization of this report during the delib­erations in Chile. Also the subject of defining figures of merit led to many livelyconver'sations. All in all, the meeting oJ the WG was very produetive and therewas a broad consensus on almost all issues discussed. The main recommenda­tions oJ the panel, Jor'm1tlated at their' {last} meeting in Antofagasta on Sep. 29are as follows:

1. Based on the limited observations presently available, La Montura does notappear' to be a viable site for the VLT. This both beeause of the poor seeingcompar'ed with Paranal, and betause of the variability oJ the seeing, whiehis cspeeially detrimental for interferometrie imaging. The image quality de­ter'ioration appears to be related to local topologieal Jeatures about whiehnothing can be done.

2. The panel was both sUT']Jrised and impressed by the apparently better seeingr'csults obtained in the Las Campanas site tests. IJ this difference is real, itindicates that there are indeed sites in Chile with seeing signifieantly betterthan 0.9 arcsec. As good seeing is crueial Jor most astronomieal observa­tions, such a large differenee would indicate the need to seareh for a bettersite Jor' the VLT. It is therefor'e uTyent to verify the Las Campanas obser­vations using an ESO seeing monitor. This should be done with the seeingrnonitor' pTcsently loeated on La Montum, so that both a eross-calibrationwith the Las Campanas monitor is possible, as well as simultaneous mea­sur'ements at Las Campanas and Vizeachas using identieal monitor's.

3. The panel inspeeted Armazoni both from the air and from the ground (al­though they did not go to the summit). The SSWG was very impressed byAr'mazoni - its prominenee, height, shape, and impor'tant area apparentlyavailable at the top. The panel proposes to terminate the La Montum sur­vey and place the La Alontura site testing equipment on Armazoni as soonas possible. It is understood that this will take at least three months, du ringwhich time the La Montura seeing monitor can be used at Las Gampanasand Gerro Paehon. Six months of simultaneous observations at Armazoniand Pamnal, extending from January to July 1990, should suffiee for eom­parative evaluation oJ the two sites without delaying the VLT project.

The SSfiVG cannot yet dmw final eonclusions about the relative merits of Paranaland Vizeaehas. It was however impressed by the dryness and sky clarity of theParanalr'egion. Its relative proximity to Antofagasta provides logistic support andcommunications at least as good as exist at La Silla. The possibility of quantify­ing the relative scientific merits of the sites was discussed, and will be explored

CIIAPTER 1. INTRODUCTION 6

further. "In fact this has now been done through the definition of figurcs of meritfor the Paranal and La Silla sites (see chapter 6). In the meantime, the removalof the seeing monitor from La Montura, its installation on La Silla, then on LasCampanas, then on Armazoni have been performed. This illustrates the excellcntcollaboration between the ESO staff and the SSWG, that, as chairman of theSSWG, I wish to stress. I also wish to express my thanks to the members of theSSWG, in particular to Marc Sarazin for his tremendous work as organizer of thesite measurements and as secretary of the working group, and to H.-E. Schusterand his team for their rcmarkable work in Chile.

Jcan-Pierre SwingsChairman SSWGMay 1990

Chapter 2

THE ESO VLT SITEEVALUATION - fromLarge-Scale Surveys to FinalComparisons of Site Parameters

2.1 Background

When the European Southern Observatory (ESO) was created, in 1962, the ex­plicit target for its future observatory site was a highly selected place in theSouthern Hemisphere (Heckmann, 1967; Blaauw, 1970, 1988, 1989a, b, c, d,1990a, b). For this reason, already in 1956, site evaluation work was initiatedin South Africa (Siedentopf, 1955; Dommanget, 1958). Thc corrcsponding in­vestigations led to identification and evaluation of a number of site candidates(Mayer, 1967).

Approximately simultaneously, astronomers from universities in the UnitedStates commenced studies of candidate places for observatory sites in Australiaand South America. At an early stage, these studies concentrated on Chile (Stock,1965).

Close collaboration was established between the site evaluation groups in SouthAfrica and Chile. Among the efforts at coordination, emphasis was given tosimilarity of instrumentation and methods far analysis.

In its first phase, the site evaluation campaign led by astronomers from theUnited States concentrated on the neighbourhoods of Santiago de Chile. As aresult, four site candidates were identified, Alto deI Toro, Cerro Colorado, CerroEI Roble and Cerro Tabaco. Comparison of observational data terminated in theselection of Cerro EI Roble, which still serves as an observing station for theChilean National Observatory, based on Cerro Calan in Santiago.

During the site evaluation campaign leading to the identification of Cerro EI

7

CHAPTER 2. THE ESO VLT SITE EVALUATION 8

Roble, it became increasingly obvious that an extension of the site survey to­wards the North offered highly interesting possibilities. Accordingly, Stock andcollaborators from the United States and from Chile initiated a second phaseof their survey. Taking into account both geographical data and the wish not 1,0

jeopardize observations of the Magellanic Clouds by going 1,00 far North, renewedsite evaluation activity was concentrated 1,0 the Elqui vallcy.

In the region surrounding the Elqui valley, foul' site candidates were dcfined,Guamayuca, Cerro 1'01010, Cerro Morado and Cerro Blanco. At the same time asobservations showed these summits 1,0 have characteristics significantly superior1,0 the summits in the neighbourhoods of Santiago, it was strongly feIt that testingof places even furt her North in Chile was of high interest.

In a third phase of their sitc survey campaign, Stock and collaborators in­vestigated mountains in the neighbourhood of Copiapo. Here, two summits wereselected, Cerro Checo and Cerro La Peineta. Approximately simultaneously somemountains in the far Chilean North were also inspected. These mountains were,however, discareled 1'01' reasons of excessive remoteness and/or lack of water andol,her facili ties necessary 1'01' infrastructural support.

As a reslilt of site tesl,ing, the grollp headed by Stock narrowed elown the pos­sibilities 1'01' the observatory to be construeted for the Association of Universitiesfor Research in Astronomy (AURA). Finally, the real choice was between Cerro1'01010 dose 1,0 the Elqui valley anel Cerro La Peineta in the neighbourhood ofCopiapo. In the end of 1962, the former was seleeted for the Cerro 1'01010 Inter­American Observatory (CTIO).

Frol11 1961, collaboration between the site survey groups from the United Statesanel Europe became very dose. Realizing the great potential of Chilean mountainsummits as observatory sites, ESO astronomers commenceel site survey work inChile.

The site candidates identified by ESO astronomers in South Africa were of highquality. Accordingly, in order 1,0 be of significant interest, Chilean site candidateshad 1,0 be at least as good. For this reason, the work of ESO astronomers in Chilewas from the beginning concentrated on mountain summits around the Elquivalley and further North. During this work it became increasingly evident thatthe part of Chile investiga1,ed offered site candidates superior even to the verybest ones identified in South Africa.

Early in 1964, it was finally decided 1,0 ereet the European Southern Obser­vatory in Chile. Somewhat later in 1964, Cerro La Silla was seleeted as the site1'01' the observatory. Construetion at La Silla was initiated in 1965 (Heckmann,1965, 1966).

CHAPTER 2. THE ESO VLT SITE EVALUATION

2.2 Experience üf La Silla as a Site

9

At La Silla, scheduled observations of scientific programmes was commencedin 1967. Accordingly, firm experience of La Silla as an observatory site nowspans elose to 23 years with a steadily increasing numbcr of telescopes of rapidlyrising technological perfection. Today, the Observatory ineludes 15 telescopes, a11heavily scheduled.

Naturally, a total of elose to 100,000 observing nights with a variety of tele­scopes and ancillary instrumentation provides a solid basis for conelusions regard­ing site quality. In addition, data explicitely aimed at evaluation of site qualityhave been collected at La Silla, especia11y during the earlier phases (Muller, 1966,1968a, 1968b, 1969; Westerlund, 1971, 1972, 1974; Ardeberg et al., 1990a).

Below, a more detailed evaluation of La Silla as a site is made, taking intoaccount large quantities of data designed to serve for site evaluation and selection.For this reason, we limit ourselves at this moment to state that, from the resultsof scheduled obscrving programmes, La Silla has been established as among thehandful of best sites for optical astronomy worldwide.

2.3 A Site für the ESO VLT

Bctween 1970 and 1980, a number of telescopes of four-metrc-elass and of rel­atively elassical design were completed for major observatories in the Northernand Southern Hemispheres. Partly in parallel, between 1975 and 1980, possibili­ties for construction of a new generation of telescopes got increasing momentum.On the one side, it seemed obvious that the new generation telescopes could bedesigned and construeted both much superior and, at the same time, cheaperthan existing predccessors. On the other side, new technologies in optical andmechanical design and construction provided attractive means to enlarge existingapertures considerably.

Taking active part in discussions concerning Very Large Telesc.opes (VLT) al­ready from the very beginning, ESO initiated a cohcrent programme for corre­sponding investigations in 1977 (Woltjer, 1983). Following the creation of a VLTStudy Group early in 1981 (Swings, 1983), efforts were increased in a numberof ways. Among the topics stressed, the siting of the ESO VLT was regarded asbeing of paramount importance (Woltjer, 1983).

Accordingly, highest priority was given to the identification of a site of the bestpossible quality.

2.4 First Site Survey

With technical planning for an ESO VLT progressing, it was obvious that aprogramme for site survey, evaluation and selection was a matter of considerable

ClIAPTER 2. TIIE ESO VLT SITE EVALUATION 10

urgency. First of all, strategies and methods had to be defined and developed.At the same time, early commencement of basic studies was a clear necessity,in order to avoid a biased site selection due to inadequate time coverage. Theimportance of an early start of the site survey programme was emphasized bythe existence of long-term trends in meteorological parameters influencing theperformance of sites for optical telescopes.

For the reasons explained, an early first site survey was initiated already in1982. Efforts included a study of satellite pictures of large-scale cloud cover andof the adequacy of such pictures for investigations of cloud cover at mountainsummits suitable for telescope sites (Ardeberg, 1983). Further, a survey was madeof all literature found relevant to the topic of site climatology. Finally, some effortswere invested in a first visit to a number of the places judged interesting for sitecandidates for the ESO VLT.

It was soon found that satellite pictures of cloud cover, whilst very adequate fora first orientation, do not serve for identification and evaluation of telescope sites.There are a number of reasons for this. First, satellite pietures are normally takenduring daytime. With highest emphasis on night time conditions and with diur­nal variations of cloud cover often being substantial, daytime satellite picturesare of limited value. Second, for any given region, satellite pictures are normallytaken at frequencies inadequate for purposes of site evaluation. Third, the wave­length bands employed are normally not very suitable for our purposes, which isespecially serious for the few night time pictures available. Fourth, spatial reso­lution of satellite pietures is nearly always too low for site evaluation purposes.Whilst most interesting mountain summits have dimensions typically less thana few hundred metres, satellite pictures normally have resolutions from a coupleof kilometres up to several kilometres (Ardeberg, 1987). For these reasons, it wasdecided to use satellite pictures only for a first preliminary identificatioIi of inter­esting regions. For later evaluation and more detailed monitoring of cloud cover,direet visual inspeetion was judged far more reliable. It is noted that for the siteevaluation programme for the United States National New Tcchnology Telcscope(NNTT) on Mauna Kea and Mount Graham (Merrill, 1987; Merrill and Forbes,1987) identical conclusions were drawn. Further, it should be added that, forpractical reasons, we chose to refrain from cloud cover monitoring through use ofradiometers, multiple-object photoelectric photometry and large-field photome­try (Ardeberg, 1987). Again, conclusions of the NNTT site evaluation group werethe same as ours.

Limiting our efforts, at least initially, to Chile, our studies of cloud cover fromsatellite pictures, our survey of literature relevant to site climatology, and our insitu studies showed good agreement. It seemed rather evident that, if we wanteda site as favourable as possible from a scientific point of view, only places Northof geographical latitl1-de -31 degrees were desirable. With Cerro Tololo at -30degrees and La Silla ~nd Las Campanas at -29 degrees geographicallatitude, it

CHAPTER 2. THE ESO VLT SITE EVALUATION 11

was obvious that an investigation of potential sites further north was potentiallyvery interesting.

2.5 Large-Scale Identification of Site Candidates

Guided by the tentative results of the first site survey of Chile performed in1982, a larger-scale effort was initiated in the first part of 1983. The strategywas rather deal', aiming at both reliable measurement procedures and a timingas favourable as possible (Ardeberg and Lindgren, 1984a, b).

For our large-scale survey of Northern Chile for VLT site candidates, some sitequality parameters were given special attention. Such parameters were cloud­cover statistics, records of integrated content of water vapour in the atmosphereand data on atmospherie turbulenee 01' seeing. Other site quality parametersidentified as important were wind eonditions (speed and direetion of wind), loealrelative humidity, temperature and temperature stability and extinetion. Further,eonsiderable attention was given to site topography, to altitude above prevailingatmospherie inversion layer and to light and dust pollution, natural as well asartifieial. Additional site parameters were eonsidered hut given lower relativeweights.

It seemcd obvious that the site quality parameter demanding the most sophis­tieated instrumentation was atmospherie turbulenee. Accordingly, it was decidedto initiate a special programme for proper development of instrumentation androutilles for analysis of observations dedieated to a study of atmosphcric turbu­lenee.

At the same time, it seemed evident that statisties regarding cloud cover andintegrated content of water vapour in the atmospherc were both very impor­tant and vulnerable with reference to long-term variations. Similar conclusionswere made for statisties eoneerning wind conditions, loeal relative humidity andtemperature and temperature stability. On the other hand, these site qualityparameters eould be measured with instrumentation less sophistieated than thatneeessary for monitoring of atmospherie turbulenee. In this context, special cau­tion was due coneerning integrated eontent of water vapour in the atmosphere,for whieh specialized and reliable instrumentation is necessary, especially whennight time data are given priority.

Measurements of integrated content of water vapour in the atmosphere weremade with sky radiance monitors. These monitors are essentially portable pho­tometers for infrared wavelengths constructed for use in the field. With the prin­cipal measurement bands at around 20 and 27pm, the monitors measure skyradianee over a fixed solid angle. Via appropriate correetions, the sky radianceis converted to sky emissivity. Applying atmospheric models, the correspondingamount of atmospheric water vapour can be derived. Out of a total of sevenInfrared Sky Radiance Monitors construeted at Kitt Peak (Morse and Gillett,

CIIAPTER 2. THE ESO VLT SITE EVALUATION 12

1982; Merrill, 1985), ESO obtained, through a programme of collaboration, two,available from early 1983. Later, a third water vapour monitor was given to ESOon a temporary basis.

Because of the principle for measurements and the wavelength bands takeninto account, the sky radiance monitors can be used at any time, in the nightas weIl as in the day. The only prerequisite necessary is a sky reasonably freefrom clouds. These characteristics define the sky radiance monitors as superiorinstruments for our purpose (Ardeberg, 1987).

With access to the Kitt Peak sky radiance monitors and with commerciallyacquired instruments such as anemometers, hygrometers, psychrometers andbarometers, we were ready to commence a large-scale in situ survey for can­didate sites for the ESO VLT in Northern Chile. This work was initiated withno further time loss.

Within a few months, a first survey was made of a large number of potentialsite candidates. The survey covered originally all of Chile between geographicallatitudes -31 and -20 degrees, although, following first impressions, .the zonesurveyecl more in detail was limited to between geographical latitudes -25 and-20 degrees. With respect to geographicallongitude, the complete range coveredby Chile was included, from the coastal mountains to summits belonging to thehigh Cordillera along the East border of Chile (Ardeberg, 1986).

As a result of the first large-scale site survey of Northern Chile, a total ofaround 30 mountain summits were identified as being of special interest for fur­ther studies. At the same time, a smaller number of summits were given highestattention, including detailed and frequent series of measurements of site qualityparameters.

2.6 Paranal

For a number of reasons, the chain of coastal mountains situated between thetowns of Antofagasta and Taltal and between the coastalline and the old Panamer­ican highway, now, except for activities of ESO, a dormant dirt road, seemedparticularly interesting (Fig. 2.1). These mountains were central in the area oflowest cloud cover as identified from satellite pietures. They were quite dose tothe coast. Further, the climate of the region was extremely dry with vegetationcxisting only in rare and protected places. In addition, being part of coastalmountains, the highest peaks had impressive heights. FinaIly, scattering in theatmosphere seemed extremcly low.

The coastal mountains between Antofagasta and Taltal were given early atten­tion. We iclentified four summits as useful telescope sites. From South to North,the summits were Para ave, Paranal, La Chira. and the ridge of Sierra Remiendos,spanning a total qistance of around 50 kilometres in the South-North direction.Of these summits~ Paranal shows the highest contrast relative to neighbouring

CHAPTER 2. THE ESO VLT SITE EVALUATION 13

CO.CO. ARMAZONES

PARANAL 10. 3064

Ir- ~AIRSTRIP2664 POSSfBLE

_." - - !

, l SllLA

[LA SE RE A·,i

lndPANAMERICAN

HIGHWAY

o SO km

Figure 2.1: Geographical position of Paranal.

CIIAPTER 2. TIIE ESO VLT SITE EVALUATION 14

mountains and also the most favourable shape. For these reasons, Paranal wasat an early stage given special attention (Ardeberg et al., 1986c, 1987).

2.7 Armazoni

Identified only slightly later than Paranal, Armazoni was quickly recognized asa site candidate of considerable interest. Compared to Paranal, it had a slightlylarger distance to the coast. On the other hand, it was significantly higher, al­though this was partly compensated by a general surrounding being higher thanthat of Paranal. The summit of Armazoni was considerably larger than that ofParanal, at the same time as the disposition of summit surface, being a North­South ridge, was not exaetly ideal. The very sparse vegetation at Armazoniindicated a dimate similar to that at Paranal. It was early decided to· pay con­siderable attention to Armazoni. The small distance to Paranal and the overallsimilarity between Armazoni and Paranal implied that reliable monitoring of thesite quality of Armazoni could be based, at least partly, on frequent intercom­parisons of parameters for the two site candidates.

2.8 Other Coastal Mountains

In the coastal region between Antofagasta and Taltal, Paranal and Armazoniwere the only summits with charaeteristics considered interesting enough for afull scale monitoring eIfort. Some lower summits very dose to the coast wereinvestigated to some extent. However, it was soon found, that, with respcet tosurrounding formations, the pattern of prevailing winds was less favourable forthese sites than for Paranal and Armazoni. For this reason, no monitoring ofseeing was made for the lower sites.

In Northern Chile, the coastal mountains outside the chain between Antofa­gasta and Taltal contain very few summits with altitudes above 2000 metres.Thus, during the first year of large-scale site surveying, Paranal and Armazoniremained the only truly coastal site candidates regularly monitored. Somewhatlater, a third coastal mountain summit was included in the list of candidates ofspecial attention. This was Tolar, a summit dose to Tocopilla, a small coastaltown between Iquique and Antofagasta. No further coastal summits wcre inves­tigated in detail.

2.9 Inland Mountains

In addition to the coastal mountains, a number of inland mountain summits wereidentified as interespng for further investigations. These summits were located inareas with impressively deal' sky, had elevations significantly higher than those

CHAPTER 2. THE ESO VLT SITE EVALUATION 15

of surrounding formations and peak shapes judged favourable in terms of tur­bulenee. It should be noted that, in inland areas, many mountains otherwisefavourably shaped and plaeed had to be excluded beeause of unaeecptable sum­mit erowding.

All inland sites included in our programme of repeated monitoring had altitudesabove 01' around 3500 metres with the highest ones reaehing above 6000 metres.Among the inland summits most frequently visited and investigated were SierraAspera, Punta deI Viento, Volcan Apagado, Volcan Taeora and Aueanquilcha.Whilst Sierra Aspera exhibits sparse vegetation at its summit, the other summitsare eompletely void of vegetation. All of them have summits whieh are either large01' at least reasonably large yet not alarmingly large in terms of possible internalturbulenee generation.

Compared to the eorresponding coastal summits, inland sites seleetcd for eon­tinued monitoring showed somewhat different eharacteristies. Photometrie skyquality tended to be somewhat less favourable, espeeially so during daytime andearly evening time. For a given geographicallatitude, this effeet had an inereasingtrend with distanee from the eoast. At thc same time, it secmed obvious that theinland sites, at least the higher ones, had an integrated eontent of water vapourwhieh was very low. Further, effeets of seattering in the atmospherc were verysmall, as judged from visual inspeetion. Hard winds oeeurrcd but not with verygreat frequeney.

2.10 First conclusions

The large-seale site monitoring and eomparison was eonductcd from 1983 to1987. It included a total of threc eoastal sites and five inland sites, plus La Silla,eovered with detailed measurements. Espeeially in thc first part of the surveyperiod, another dozen site eandidates were included but sueeessively eliminated.Sueh elimination was based on observational data in all eases with one exeeption,in whieh ease aeeessibility was severely deteriorated through alandslide bloekingan aeeess trail at lower level. Further, in the last part of the survey period, alldata for Tolar had to be eolleeted somewhat below the summit. This was enforeedbeeause of military mining of the summit area proper.

By the end of 1986, it was deeided to eoneentrate final efforts on thc coastalsites Paranal and Armazoni, at the same time as some summits surroundingParanal were given inereased attention. The coastal sites were preferred for avariety of reasons, including the signifieantly more favourable photometrie skyquality, generally lower wind speeds but also due to the overall aeeessibilitybeing eonsiderably more easy than for the inland sites. Among the coastal sites,Tolar was eliminated on aeeount of its eomparatively high level of integratedatmospherie water vapour but also due to its proximity to Toeopilla.

In the neighbourhood of Paranal, some summits had been identified as poten-

CHAPTER 2. THE ESO VLT SITE EVALUATION 16

tially promising, either for a large telescope array 01' for installation of singletelescopes. Being only somewhat lower than the summit of Paranal proper, thesesummits offered attraetive perspectives as alternative and/or auxiliary sites.Thus, it was decided to make a differential study, comparing surrounding sum­mits to that of the principal peak.

There can be little doubt concerning the high quality of Paranal and Armazoni,ineluding surrounding summits, as sites for telescopes operating in the opticaland infrared wavelength regions. At the same time, the results of our site surveysleft no doubt regarding the interesting prospeets offered by the high inland sitesstudied in more detail. It must be regretted that circumstances did not permita eloser investigation of these summits.

2.11 Sites outside Chile

From its initiation, the survey for an ESO VLT site had concentrated on thenorthern part of Chile. At the same time, also other possibilities were discussed.In this context, it was early concluded that alternatives of competitive qualitywere not abundant in the Southern Hemisphere. Such alternatives could, possibly,be found on some islands with higher mountain summits, in southern Peru andin some parts of Namibia.

Relying on earlier comparisons between summits in Chile and in southernAfrica as well as on the experience of German groups in Africa, it 'was con­eluded that Chilean sites were superior to those offered on the African continent.Regarding possible site candidates in Peru, there was no extensive experienceto rely on. However, from inspection of satellite pictures and general evidence,it seemed strongly indic.ated that in Peru only higher summits could be takenas serious competitors to Chilean site candidates. The reason for this conclusionwas the increase of eloud cover with decreasing southern latitudes. This being thecase, it was obvious that limitations for Peruvian sites would be much thc sameas for the inland Chilean sites. In addition, it seemed clearly indicated that theselimitations would be at least as pronounced for the Peruvian site candidates asfor the corrcsponding summits in the northern part of Chile.

As a result, the superiority of Chilean sites was found obvious, with the pos­sible exception of island sites. A survey of possible candidates for island sites,with consultance of earlier experience (Walker, 1987), made us conclude, thatthe candidate by far most viable was defined by the mountain summits on LaReunion. These summits had a number of favourable features in addition to theirstatus as island sites. Mention may be made of latitude, altitude and contrastagainst the surrounding landscape. For this reason, it was, in the first part of1986, decided to include La Reunion in the site selection campaign for the ESOVLT. !

CHAPTER 2. THE ESO VLT SITE EVALUATION

2.12 La Reunion

17

The only mountain summit situated outside Northern Chile and seleetcd as acandidate site for the ESO VLT was Grand Benare on the island of La Reunion.Starting in August, 1986, detailed monitoring was made until and ineluding May,1987. Parameters studied were eloud cover, amount of intcgrated atmosphericwater vapour, wind and local relative humidity plus precipitation.

Thc summit of Grand Benare has an altitude above sea level of 2896 metres.The area of the summit is relatively generous. Vegetation is limited but notmarginal The highest neighbouring peak is the Piton des Neiges. This peakhas an altitude of 3070 metres and is situated approximately six kilometres tothe East-North-East of Grand Benare, with the Cilaos crater in between the twosummits. Whilst lower than Piton des Neiges, Grand Benare has a softly roundedsummit, highly contrasted to the rugged shape of the Piton des Neiges summit.

Access to the Grand Benare summit goes via a most reasonable trail, use ofvehicles being excluded. Artificial light pollution is very limited at the summit.Approximate geographical coordinates are 55°25'45" E and -21°06'1 0" S.

References-Ardeberg, A. 1983: Site Selection /or a Very Large Telescope, in Workshop onESO's Very Large Telescope, Cargese, 16-19 May, 1983, eds. J.-P. Swings andK. Kjaer, ESO Conf. and Workshop Proc. 17, p. 217-Ardeberg, A. 1986: ESO VLT Site Evaluation, in ESO Conf. and WorkshopProc. 24, Second Workshop on ESO's Very Large Telescope, Venice, 29 Sept.-2Oet., 1986, eds. S. D'Odorieo and J.-P. Swings, p. 221-Arcleberg, A. 1987: Modern }'!fethods 0/ Site Testing, and Proe. of the Int.Conf. on "Identifieation, Optimization, and Proteetion of Optieal Sites",Flagstaff, Arizona (May 22-23, 1986). R.L. Millis et al., edit., LowellObservatory, Flagstaff (1987), p20-Ardeberg, A. and Lindgren, H. 1984a: First results /rom initial site testing /ora V.L. T in Chile; Proe. Eso Workshop on "Site testing for Future LargeTeleseopes"; La Silla, 4-6 Oet. 1983. Ed. A. Ardeberg, L. Woltjer, Eso proe.n018 p. 151.-Ardeberg, A. and Lindgren, H. 1984b: Some Possible VLT Sites in Chile, inProe. IAU Coll. 79, Very Large Teleseopes and their instrumentation andprograms, Garehing, April 9, 12 1984, Ed. M.-H. Ulrieh and K. Kjaer, ESO-Ardeberg, A., Lindgren, H. and Lundström, 1. 1985: ESO VLT Report 43-Ardeberg, A., Lindgren, H. and Lundström, 1. 1986a: ESO VLT Report 45-Ardeberg, A., Lindgren, H. and Lundström, 1. 1986b: ESO VLT Report 53-Ardeberg, A., Lindgren, H. and Lundström, 1. 1986e: On the Relative Merits 01Some Possible VLT sites in Chile in ESO Conf. and Workshop Proe. 24,Seeond Workshop on ESO's Very Large Teleseope, Veniee, 29 Sept.-2 Oct.,

CIIAPTER 2. THE ESO VLT SITE EVALUATION 18

1986, eds. S. D'Odorico and J.-P. Swings, p. 221-Ardeberg, A., Lindgren, H. and Lundström, 1. 1987: ESO VLT Report 54-Ardeberg, A., Lindgren, H. and Lundström, 1. 1990a: Astron. Astrophys., 230,518-Ardeberg, A., Lindgren, H. and Lundström, 1. 1990b: ESO VLT Report-Ardebcrg, A., Lindgren, H., Lundström, I. and Sarazin, M. 1987: Progress ofthe site testing for the ESO VLT; Proc. of the Int. Conf. on "Identification,Optimization, and Protection of Optical Sites", Flagstaff, Arizona (May 22-23,1986). R.L. Millis et al., edit., Lowell Observatory, Flagstaff (1987) p. 94.-Blaauw, A. 1970: ESO Ann. Rep. 1969, p. 5-Blaauw, A. 1988: The Messenger, 54, 1-Blaauw, A. 1989a: The Messenger, 55, 14-Blaauw, A. 1989b: The Messenger, 56, 21-Blaauw, A. 1989c: The Messenger, 57, 39-Blaauw, A. 1989d: The Messenger, 58, 24-Blaauw, A. 1990a: The Messenger, 59, 27-Blaauw, A. 1990b: Thc Messenger, 60, 23-Brault, J.W., Fender, J.S. and Hall, D.N.B. 1975: J. Quant. Speetrosc. Radiat.Transfer 15, 549-Dommanget, J. 1958: Ciel et Terre 74, 305-Flower, T.F. 1974: Thesis, University of Wyoming, U.S.A.-Forbes, F.F., Morse, D.A. and Poczulp, G. 1986: Planning the National NewTechnology Telescope (NNTT): VI. Site survey instrumentation, NOAO NNTTTechnology Development Program Report 8, p. 52-Fuenzalida, H. 1984: Meteorological Conditions in N01'thern Chile, Proc. ESOvVorkshop on ".Site testing for Future Large Telescopes" ; La Silla, 4-6 Oct.1983. Ed. A. Ardebcrg, 1. Woltjer, ESO proc. n018 p. 151.-Hcckmann, O. 1965: ESO Ann. Rep. 1964, p. 5-Hcckmann, O. 1966: ESO Ann. Rep. 1965, p. 5-Heckmann, O. 1967: in Astronomical Site Testing in South Africa, ed. U.Maycr, ESO Publ. Tübingen, p. 5-Maycr, U. 1967:Astronomical Site Testing in South Africa, ESO Publ.Tlibingen-Mcrrill, K.M. 1985: NOAO/NNTT Site Evaluation Projeet Report onInst1'umentation: Infrared Sky Radiance Monitor-Merrill, K.M. 1987: NNTT Site Evaluation Projeet: An Overview, in Proc.Intern. Conf. on Ident., Optimiz. and Proteetion of Opt. Tel. Sites, Flagstaff,Arizona, May 22-23, 1986, eds. R.1. Millis, O.G. Franz, H.D. Ables and C.C.Dahn, Lowell Obs., Flagstaff, Arizona, U.S.A., p. 30-Merrill, K.M. and Forbes, F.F. 1987: Comparison Study of Astronomical SiteQuality of l\i[ount qraham and Mauna J(ea NOAO NTT Technologydevelopment program Report nOlO, p. 1.

CHAPTER 2. THE ESO VLT SITE EVALUATION 19

-Moore, C.E., Minnaert, M.G.J. and Routgast, J. 1966: The Solar Speetrum2935 to 8770 ,Second review of Roland's preliminary Table of the solarspeetrum wavelengths. National Bureau of Standards monograph 61-Morse, D. and Gillett, F. 1982: AURA Eng. Report 73-MuHer, A.B. 1966: ESO BuH. 1, 16-MuHer, A.B. 1968a: ESO BuH. 3, 45-MuHer, A.B. 1968b: ESO BuH. 4, 23-MuHer, A.B. 1969: ESO BuH. 7, 19-Schulz, A. 1990: Max-Planck-Institut für Radioastronomie, Div. of Subm.Techn. Mem. 121-Siedentopf, 1955: The Climate of the Union of South Africa, Astron. Inst.Tübingen-Swings, J.-P. 1983: ESO's VLT Studies and Options, in Workshop on ESO'sVery Large Telescope, Cargese, 16-19 May, 1983, eds. J.-P. Swings and K.Kjaer, ESO Conf. and Workshop Proc. 17, p. 205-Walker, M.F 1987: Charaeteristics of Optimum Sites, in Proc. Intern. Conf. onIdent., Optimiz. and Proteetion of Opt. Tel. Sites, Flagstaff, Arizona, May22-23, 1986, eds. R.L. Millis, O.G. Franz, R.D. Ables and C.C. Dahn, LoweHObs., Flagstaff, Arizona, U.S.A., p. 128-Warner, J.W. 1977: Publ. Astron. Soc. Pacific 89, 724-Westerlund, B.E. 1971: ESO Bull. 8, 5-Westerlund, B.E. 1972: ESO BuH. 9, 3-Westerlund, B.E. 1974: ESO BuH. 10, 5-Westphal, D.A. 1972: Infrared Sky Noise Survey: Final Report, NASA-CR139693, Springfield, Virginia, National Technical Information Service.-Woltjer, L. 1983: The VLT Workshop: Introduction and Conclusions, inWorkshop on ESO's Very Large Telescope, Cargese, 16-19 May, 1983, eds. J .-P.Swings and K. Kjaer, ESO Conf. and Workshop Proc. 17, p. 1-Woolf, N.J. 1968: Site Seleetion Study for the University of Minnesota, FinalReport

20

Figure 3.1: The differential motion seeing monitor telescope on its test tower at CerroVizcachas

Chapter 3

METHODS ANDINSTRUMENTS

3.1 Introduction

As explained in Chap. 2, the site evaluation campaign for the VLT started withinstruments easy to use and to transport to a large number of places in theSouthern hemisphere. The instrumentation grew more and more sophisticatedas the number of possible site candidates was reduced.

A complcte monitoring station currently includes measurements of cloudiness,sky emissivity, meteorological parameters (temperature, humidity, wind velocityand clirection), seeing, scintillation, microthermal activity at ground level and inthe boundary layer.

The sky emissivity is monitored twice an hour with a KPNO infrared radiome­ter in manual mode. In the same time, the observer notes down the status of thecloud cover.

The differential motion seeing monitors (DIMM) shown on Picture 3.1, deve­loped by ESO, are operated all night long in a semi-automatic mode, needingoperators at each change of star. The seeing is displayed on line every minute,automatically archived and may be statistically processed on the spot if required.The first DIMM started routine measurements on Cerro Paranal in April 1987,followed by a second system in September 1988 on Cerro Vizcachas, and a thirdone in April 1989 on Cerro La Montura, 2500m, 4km from Cerro Paranal.

It is also a matter of interest to quantify the contribution of the various at­mospheric layers to the total seeing. Besides the DIMM, a complete monitoringstation includes a scintillometer, and a set of microthermal sensors. One also usesan acoustic sounder for the analysis of the turbulence in the boundary layer, anda dust meter which measures the number of particles (0.3 to 10 pm in diameter)per unit volume of air.

21

CIIAPTER 3. METIIODS AND INSTRUMENTS

3.2 Evaluation of Cloud Cover

22

Following a critical evaluation of methods available for evaluation of cloud cover,it was decided that whilst intrinsically interesting, neither satellite surveys norradiometric measurements were useful in practice. For the same reason, alsomonitoring of a limited amount of selected objects and large-field imaging wereexcluded. Instead, it was decided to concentrate on visual estimates of cloudcover.

Obviously, visual estimates of cloud cover were chosen because of practicalconsiderations. At the same time as this method is rather traditional, it is highlysuitable for field work. Moreover, if careful precautions are observed, the methodcan yield rather reliable results (Ardeberg, 1987; Ardeberg et al., 1986c; Merrilland Forbes, 1987).

In order to secure adequate data from our cloud cover evaluations, a numberof steps were taken. Several persons were carefully trained. It was endeavoredto providc working conditions as decent as feasible. Long working turns wereavoidecl. Finally, critcria were defined in a transparent way.

Cloud cover observers were asked to evaluate conditions in a strict manner.Observing data included percentage of sky covered, with detailed data beingnoted for different types of clouds. Further, possible transparency of clouds hadto be evaluated and noted. Locations and movements of clouds were included.Also clouds below the level of the observers had to be noted, including the altitudeof their upper limits. As an aid, a simple yet reliable fixed pointer was providedfor measurements of elevation and azimuth angles. For identification of cloudtypes, clescriptions were provided, including pictures.

In order to enhance the consistency and reliability of cloud cover evaluations,frequent quality controls were made. These controls included independent che­cking, with and without notice to observers. It should be noted, that such controlsrevealed a high degree of consistency.

As a final check on the overall consistency of cloud cover evaluations, compa­risons were made of observing data obtained during periods of full moon and newmoon, respectively. The power of these tests was enhanced through the inclusionof daytime data, which, to a high degree, eliminates spurious comparison resultsduc to large-scale weather patterns (Ardeberg et al., 1986c).

References: see Chapter 2

CHAPTER 3. METHODS AND INSTRUMENTS

3.3 Measurements of Atmospheric Water Vapour

23

3.3.1 Introduetion

For the determination of the integrated content of water vapour in the atmo­sphere, three different types of measurement procedures were considered. Thesewere radiosonde ascents, water-vapour meters and monitors measuring sky radi­ance (Ardeberg, 1987).

Several ascents of radiosondes with humidity sensors were made. Correspondingintegration of water vapour was made to altitudes for which the content wasvery low. In spite of several precautions, the method did not turn out to be veryreliable.

The reasons for our less reliable data on integrated atmospheric content ofwater vapour from radiosonde ascents were several. Most essential, it was evi­dent that the humidity sensors showed a severe lack of precision. This was es­pecially obvious for lower ambient temperatures. As a result of the atmospherictemperature structure and the corresponding distribution of water vapour, thisresulted in rather low consistency and precision. These shortcomings are wellknown (Fuenzalida, 1984). Accordingly, data from radiosonde ascents were not,as such, taken into account for evaluations of the content of integrated atmo­spheric water vapour. However, they were used for independent checks of theresults of our monitor data, described below.

Water-vapour meters have been used extensively for determining the integratedlevels of atmospheric water vapour (WooU, 1968; Westphal, 1972; Flower, 1974;Warner, 1977). This has emphasized that the method, whilst intrinsically ratherstraightforward, has a number of weak points.

First of all, water-vapour meter can be used only in daytime. Unfortunately,diurnal variations in atmospheric content of water vapour are far from negligi­ble. In addition, large zenith corrections have to be applied, especially so forthe observing data most interesting for night-sky observations. These correc­tions are often highly spurious, as shown from our own data of short-time vari­ations of water-vapour content. Further, corresponding uncertainties are intro­duced through correetions for atmospheric pressure and allowance for absorptionin the solar continuum. Finally and most unfortunate, data from water-vapourmeters are rather uncertain for lower amounts of water vapour, induding levelshighly interesting for the site evaluation.

Compared to ascents of radiosondes with humidity sensors and to the use ofwater-vapour meters, employment of sky-radiance monitors may well be regardedas a somewhat less direet method for evaluation of the amount of integrated at­mospheric water vapour. On the other hand, with measurement bands in theinfrared wavelength region, sky-radiance monitors can be operated at any time,independent of the position of the Sun. The only conditions which have to berespected are that the sky is dear and that neither the Sun nor the Moon be

CHAPTER 3. METHODS AND INSTRUMENTS 24

allowed to shine directly down in the observing aperture of the monitor. In addi­tion, light douds, such as high cirrus, do not affect measurements significantly.

3.3.2 The sky-radiance monitors

For the ESO VLT site survey and evaluation, it was decided to employ sky­radiance monitors for measurements of the integrated atmospheric content ofwater vapour. For the NOAO/NNTT site evaluation campaign, an excellent setof sky-radiance monitors was developed and constructed (Morse and Gillett,1982; Merrill, 1985; Forbes et al. , 1986; Merrill, 1987). Out of a total of sevenmonitors constructed, ESO obtained, through an agreement on cooperation, twounits. Later, a third monitor was included on a long-term loan basis.

With the sky-radiance monitors, the intensity of the sky radiation is measuredin four bands. These bands are centered on 11.1, 14.9,20.0 and 26.7 micrometre,respectively. The corresponding bandwidths are 0.8, 1.8, 3.6 and 9.0 mi~rometre.

One of the four measurement bands (11.1 micrometre) is placed in a wavelengthregion where the atmosphere is very transparent. Thus, in this band, we obtain ameasure of sky transparency. Another of the measurement bands (14.9 microme­tre) is centered on the fundamental band of C02, which is highly saturated.With this band, a measure is obtained of the local atmospheric temperature. Atthe same time, the internal temperature of the monitor is recorded for all skymeasurements. FinaIly, the two principal measurement bands, at 20.0 and 26.7micrometre, are located in wavelength regions characterized by heavy absorptionby water vapour.

In addition to measurements through the passbands described, aseries of mea­sures made with a water vapour monitor always includes those made with thelight beam blocked. Further, all measurements are made in the form of the record­ing of the difference in flux between the radiation from the sky and that from aninternal reference source. This reference source is a blackbody surface containedin the monitor. With a pyroelectric detector, measurements are made using achopper, alternating between sky signal and reference signal. The monitor re­ceives sky light over an aperture corresponding to 0.03 steradian. Finally, forall measurement series, the temperature of the internal blackbody reference isrecorded.

Observations of the sky can be made in any position. Except for control mea­surements, we consistently made our observations in the zenith position.

The water vapour monitors are portable field instruments run on internal bat­tery packs. Operating in the thermal infrared wavelength region, they can beused at any time in the day as weIl as during night time. The only precautionsnecessary are that the sky is reasonably dear and that neither the Sun nor thefull Moon are allo\Yed to shine directly down in the aperture. All monitors requirethe presence of an operator.

CHAPTER 3. METHODS AND INSTRUMENTS

3.3.3 Calibrations

25

Running calibrations of the sky-radiance monitors are made from measurementsof liquid nitrogen. Corresponding absolute calibrations are made via high-resolutionspeetroscopy of absorption lines of water vapour.

For the running calibrations with liquid nitrogen, cryogenic dewars are avail­able. These are filled and refiIled several times, aIlowing for proper adjustmentof the temperature of the dewar. FoIlowing this, the dewar is placed over theaperture of the monitor, and a fuIl series of measurements is made, includingaIl measurement bands. In this way, we use the temperature of liquid nitrogenas reference. Calibration with nitrogen is made with the monitor at a variety ofdifferent temperatures, covering those used for programme measuremcnts. Thegreat majority of such measurements have been made at ambient and monitortemperatures between -15 and +30 degrees Celsius, with only a smaIler fractionfalling outside this interval.

Whilst highly adequate for running calibrations and check of consistency, mea­surements of liquid nitrogen do not provide a reliable means of absolute cali­bration of measurements of the sky made with sky-radiance monitors. For thispurpose, we relied on high-resolution spectroscopy of lines of atmospheric watervapour. Thus, these latter measurements constitute our ultimate reference for ab­solute calibration, whilst the measurements of liquid nitrogen serve to calibrateour temperature corrections and monitor consistency of programme measure­ments.

For our absolute calibrations of the sky-radiance monitors, rather strict pro­cedurcs were always adopted. Simultaneous observations were made with theCoude EcheIle Spectrometer (CES) coupled to the Coude Auxiliary Tclescope(CAT) and one 01' more of the sky-radiance monitors. The monitors were locatedat some level above ground, elose to the CAT building. All measurement bandswere measured in a rotating manner.

In the mode employed, using a Reticon deteetor, the CES gave a spectral reso­lution elose to 105 . We recorded a spectral region with a width of approximately5 nanometre, centered on the water-vapour line at 694.3807 nanometre.' This is arather well-studied line with an adequately determined behaviour and oscillatorstrength (Moore et al., 1966; Brault et al., 1975).

After appropriate reduetions, the measurements obtained with the sky-radian­ce monitors provide data on sky emissivity, one referring to 20.0 micrometre,the other to 26.7 micrometre. These emissivity data can be converted into corre­sponding amounts oE atmospheric precipitable water vapour. For this conversion,an atmosphcric model has to be employed. We adopted the same model as usedfor the NOAOjNNTT site evaluation campaign (Merrill, 1985). It should benoted that this model is only provisional.

Our absolute calibrations cover rather weIl the interval oE water-vapour con­tent most interesting for site evaluation. Calibration measurements with the CES

ClIAPTER 3. METHODS AND INSTRUMENTS 26

range from below one to more than six millimetre of precipitable atmosphericwater vapour. For both sets of monitor data, referring to 20.0 and 26.7 microme­tre, respectively, linear calibration relations are obtained as expressed in amountof precipitable atmospheric water vapour. In both cases, the spread around theserelations is quite reasonable. It should be noted, that inhomogeneities in the dis­tribution of water vapour in the atmosphere are responsible for a significant partof this spread.

For the measurement band situated at 26.7 micrometre, the resulting calibra­tion line is very elose to a one-to-one relation. The corresponding line for theband centered at 20.0 micrometre deviates considerably from a one-to-one rela­tion. At the same time, the relation between the data for the two measurementbands is rather tight. Thus, we have chosen to use the calibrated data for theband at 26.7 micrometre as reference, whilst for every measurement of the sky, weuse the average data as obtained from the two bands. The only exception is foramounts of precipitable atmospheric water vapour in excess of 8 millimetre. Forthese levels, the measurement band at 26.7 micrometre saturates. Accordingly,for correspondingly high amounts of precipitable water vapour, we have adoptedthe values from the band at 20.0 micrometre, calibrated on the scale defined bythe band at 26.7 micrometre.

From the spread around the calibration relations for the two measurementbands as weH as from the corresponding spread in the relation between the twobands, we have estimated the accuracy of our final data. For levels of integratedprecipitable atmospheric water vapour up to approximately 3 millimetre, the re­sulting dispersion in a single determination of precipitable water vapour does notexceed 0.3 millimetre. For corresponding levels between 3 and 5 millimetre, theresulting dispersion does not exceed 0.5 millimetre. Also for levels of precipitablewater vapour up to around and even somewhat exceeding 8 millimetre, our dataindicate very reasonable dispersions. For atmospheric water-vapour contents sig­nificantly in excess of 8 millimetre H20, the dispersion of our measurement datais more difficult to estimate.

The deviation of the calibration line for the measurement band at, 20.0 JLmfrom a one-to-one relation strongly indicates that the provisional atmosphericmodel adopted is not fuHy correct. Whilst this has no infiuence on data on skyemissivity, corresponding data on precipitable water vapour may be infiuenced.Such infiuence should, however, in praetice, be elose to negligible, due to ourabsolute ralibration using high-resolution spectroscopy.

Strietly speaking, our absolute calibration should be valid only for altitudesvery elose to that of La Silla, where the calibration was made. However, giventhe small difference in altitude between La Silla and Paranal, it seems highlyprobable that the calibration is, in practice, also valid for the data obtainedat Paranal. This :ineludes the effect of the slightly different latitude of the twosummits. For summits with altitudes more radically different from that of La

CIIAPTER 3. METHODS AND INSTRUMENTS 27

Silla, it has to be remembered that the validity of the absolute calibration ofprecipitable atmospheric water vapour is increasingly uncertain. In such cases,site comparisons regarding atmospheric water vapour should be based on skyemissivity alone.

References: see Chapter 2

3.4 Light Pollution Sources

With faint distant objeets as one of high priority items, the site of the VLT shouldhave a night sky as dark as possible. This implies that artificial light pollutionshould be as low as possible. In general, artificial light pollution is very low inNorthern Chile. The concentration of the modest population to some very fewurban centres guarantees basic freedom from city lights in the majority of places.Moreover, the site candidates studied for the final choice of the VLT site are es­pecially favourable in this respect, being situated far from urban concentrations.At the same time, a number of mines are spread over the mountain regions in thenorthern part of Chile, often giving rise to significant light pollution. However,also in this respect, our final site candidates benefit from favourable conditionswith no significant mining aetivity disturbing.

For all site candidates, the amount of noeturnal light pollution has been esti­mated. Such estimates have taken into account stationary pollution sources suchas urban centres, mines and faetories. Further, variable sources of light pollutionsuch as major roads and air line corridors have been noted. Especially in the caseof stationary light sourees, a direct line of sight has to be avoided. At thc sametime, also indireet light pollution from light sources without a direct !ine of sightshould be avoided to the greatest extent possible.

CHAPTER 3. METHODS AND INSTRUMENTS

3.5 Seeing monitor

28

Seeing is a measure of the integrated effect of several atmospheric layers movingat different altitudes, with different speeds and even in different directions. Anideal monitor would deliver information on aB those parameters. .

Only one system allows to remotely determine the altitude of the turbulentlayers, it is the Scidar (Scintillation Detection and Ranging) [1]. It is based onthe spatio-temporal analysis of shadow patterns produced by double stars in thepupil plane of a large telescope. Though a transportable version with a plasticrefractor has been successfully tested, it remains a rather heavy instrument dueto the large aperture (80cm) required.

On the other hand, speckle analysis is the only optical method which canprovide information on most parameters [2] but only with large apertures (2m)and at the cost of much computing time.

After a proposal by F. Roddier, it was decided to build a differential imagemotion monitor (DIMM) as the main tool for the measurements of seeing qua­lity. One uses two circular apertures in the pupil plane (Hartmann holes) andmeasures in real time the standard deviation of the differential motion of the twoimages in two perpendicular directions of the image plane.

The theoretical relations relating differential image motion to long exposureimage size are summarized in [3]. The choice of the size and separation of thesub-apertures is a compromise between sensitivity which requires small aperturesfar apart, and accuracy which sets a lower limit to their diameter. The instrumentprovides the possibility to change easily the sub-aperture configuration.

This method has several advantages since tbe telescope spurious motions aresubtracted out and because, unlike direet imaging, it does not necd a perfectoptical quality. The only constraint on image quality is the circularity of thespots for the centroiding algorithm.

Contrary to direct image size measurements which require an aperture at leasttwo times larger than the secing Fried parameter ra (ra = 20 cm for a 0.5 arcsecseeing at A= 0.5 pm), the differential motion monitor is not limitcd by the sizeof the telescope.

Bright stars are tracked two hours before and after the meridian, various sizesof pupil plane masks may be used according to average seeing quality.

The system uses an intensified CCD connected to a desktop computer througha commercial16 Bit parallel interface. The standard deviations of the differentialmotions in the directions parallel and perpendicular to the line of separation ofthe two sub-pupils are computed in real time, giving every minute an estimateof the current FWHM.

CHAPTER 3. METHODS AND INSTRUMENTS

SEEING MONITOR

DIFFERENTIAL IMAGE MOTION

29

- - - - ...... --- - -- -.--

$JL/EE=================='===:3o(~__E.d ---;~n

Pupil Plane Mask~~

CCD ImaRe Plane___-+-.:!-.j'C-- --L~""'-;l>-'---......._-~_ ...

q~ = q: ~ q;., 2::: (3.H/"'P.'D-1/3r~~/3:::O.18'\'D-1/3r~~/3

Automatie Subtraetion ofTraeking Errors and Wind Load

ql~ ::: 2(O.18D- 1/ 3 - O.097d- 1/ 3).\'r;S/3

q~ ::: 2(O.18D- 1/ 3 - O.145d- I / 3),\'rös/3

Figurc 3.2: Schematics of thc mcthod: measurement of the relative wavefront slopedifferences

CHAPTER 3. METHODS AND INSTRUMENTS

3.6 Microthermal sensor

30

(3.1)

Each candidate site for the YLT is being investigated with respect to the turbu­lence in the range 5-30 metre over the ground so as to evaluate the dependencyof the image quality with the telescope elevation from the ground [5J. Groundroughness, colour and mountain slope are supposed to affect the altitude distri­bution of the turbulence in the first meters above a summit.

A data acquisition system has been developed at ESO in collaboration withresearch institutes and is now used for quality assessment of Chilean sites.

Cold platinum wires are used for detecting air temperature variations of afew millidegrees. They must have a short response time to cope with high windconditions (a turbulence cell, 10 cm in diameter would pass through the sensorin 10 ms with a wind speed of 10 m/s.).

The sensors werc defined and tested in the Laboratoire de Thermodynamiqueof the University of Rouen (F). They have the following characteristics :

Platinum wire, diameter 3.2 Jlm, length 3 mm.

Measured temperature coefficient: 1.6510-3 C-l.

Measured 3 dB bandwidth (T = 20 C, flow velocity = 2m/s): 480 Hz.

Resistor: 110 n ± 2% (T = 20 C).

Nominal intensity: 0.3 mA.

These sensors are used with low noise operational amplifiers developed at ESO.The resolution at 100Hz bandwidth is bettel' than 5 millidegrees with an overallstatic sensitivity :

S = -~:~73 j CJYolts on a ±10 C linear range.

The turbulence features may be considered as moving horizontally at the speedof the wind which carries them in a frozen atmosphcre hypothesis. One can definean cquivalent time lag ßt = ßr /11, where 11 is the short time average windvelocity. The temperature structure parameter measured at the height H withonly one sensor is then:

C2(t H) = «T(t,H) - T(t + ßt,H))2>r . . C2 -2/3T , (ßt . u)2/3 ,111 m

Where < . >r means that the computation is made on time series long enoughcompared to ßt (typically T = 1 mn) and during which the wind velocity maybe considered as constant.

CHAPTER 3. METHODS AND INSTRUMENTS

3.7 Scintillometer

31

The index of scintillation is a non dimensional parameter defined as the nor~

malized variance of the apparent luminosity of stars. The scintillation of stars islarger when turbulence occurs at high altitude and may have a temporal band~

width up to 1000Hz according to the ve10city of the motion of the turbulentlayers.

The system used for the site evaluation campaign is a simple Fabry lens ima~

ging a 3cm diameter entrance pupil onto the photocathod of a photomultipliertube (Thorn Emi 9524B). The spectral filter is provided by the Sl1 Bialkaliphotocatode speetral response (from 300nm to 550nm with the maximum at400nm).

The output signal of 1kHz bandwidth is processed in an analog electronic boarddesigned by the Departement d 'Astrophysique of Nice university (F).

After filtering out the photon noise and subtraeting the sky background, thestandard deviation of the star intensity is computed, normalized and integratedover 60 seconds. The overall bandwidth of the measurement is around 500Hz, asobtained in the laboratory.

The field of view is limited to 30 arcmin with a diaphragm, a valuc large enoughso that such instruments can be used individually on inexpensive equatorialmounts for several hours without tracking corrections.

Scintillometers are attached to the seeing monitor telescope, the scintillationof the same star is thus monitored with a high bandwidth (500Hz) on the pho­tomultiplier and with a low one (50 Hz) on the CCD. The comparison of thosesimultaneous data sets provides an indication on the velocity of the turbulentlayers.

3.8 Acoustic sounder

Microthermal aetivity changes the property of the atmosphere with respect tothe propagation not only of light waves but also of sound waves. In particular theamount of backseattered aeoustie energy is proportional to the loeal temperaturestrueture parameter CHt, H) deseribed previously.

The aeoustie sounder or Sodar (Sodar = Sound Deteetion and Ranging) isin principle the best tool for boundary layer monitoring. It is so in faet fordiagnosis and interpretation of mierometeorologieal phenomena. Another usefulfeature of the sodar is its transportability, many subsites can be easily comparedto a referenee summit closeby. In case of large seale mountains, loeal orographiedisturbanees may be revealed in a minimum of time.

The sodar data, being only relative, are not used in the estimation of therelative quality of sites deseribed in the following paragraphs, but this instrumenthas been, and will eontinue to be, the only means to see thermal turbulenee in

CHAPTER 3. METHODS AND INSTRUMENTS

. -

!IIaJ-~,

fE

.ffi ..t{,

32

Figure 3.3: Sodar antenna and meteorological mast near the 1m telescope at La Silla

real time.Pieture 3.3 shows the antenna of the Remtech acoustic sounder installed at La

Silla at the end of 1985. Such a device sends upwards a 1600 Hz, 200ms longacoustic pulse through a parabolic reflective antenna. The backscattered echo istime gated and Doppler processed. A maximum of 20 altitude slices are available,startillg at 30m up to 800m above the site. Automatie temperature compensationof emitted frequency for constant antenna efficiency and double pulse techniquefor optimal fixed echo deteetion are standard features.

3.9 Dust meter

1 Dust pollution has long been recognised as a nuisance and sometimes a problemfor telescopes, and concern has been expressed that it may be even more seriousfor the open air configuration of the VLT. Three lines of research are been pursuedto taclde this problem:

1. Measurements and evaluation of dust concentrations in the ambient air oftelescope environments, either within domes 01' in the open air as shouldbe the case for the VLT.

1 By Paul Giordano and Lorenzo Zago

CHAPTER 3. METHODS AND INSTRUMENTS 33

2. A research on quantification of the effect of dust on mirrors, in particularwith respeet to the increase of emissivity, which is a key parameter forobservations in the infrared.

3. A study on cleaning procedures and also possible devices which will decreasedust deposition on mirrors.

The equipment used for these measurements is a particle counter CLIMETModel CI-8060 which counts and sizes particles from 0.3 pm to 10 pm. Thereare six channels of discrimination: 0.3, 0.5, 0.7, 1, 5 and 10 pm. The standardsampIe flow rate is 1 cubic foot per minute (CFM). The CI-8060 acquired byESO is equipped with a thermal printer and a RS-232 serial port for conneetionto a computer. It is joined by a multiport sampIer which allows to have up to 12lines of tube for the sequential monitoring of different locations.

The particle detection is done by an optical system: the air containing theparticles to be sampled is aspired and brought into the primary focus of anelliptical mirror. When a particle enters ihe sensing zone, it scatters light to aphotomultiplier tube located at the secondary focus which converts the opticalsignal to an eleetric one.

This equipment is normally used for cleanroom monitoring as required forinstance in the eleetronic components industry and more seldom for measuringparticle concentration in outside air, although its measuring concentration limitis at about 1.55 x 106 particles per cubic foot, which should be seldom reachedin an open environment.

3.10 Wind flow modelling

The knowledge of the wind flow pattern over ihe whole area around a site helps1,0 understand possible differences between subsites by pointing out local par­ticularities. While a wind tunnel study is rather long 1,0 implement and lacksversatility, a numerical analysis offers, with the help of recently developed com­puter codes [6], the possibility of simulating any situation by a simple changeof input conditions. This is the object of a contract between ESO and RisoeNational Laboratory (Denmark).

The several years of meteorological data available for Paranal are used to de­termine typical meso-scale conditions which are then used as input for the model.The meteorological data base of La Montura is used to check the performancesof the model.

Topographie maps are digitized on an area of 12x12 km with a 100m resolutionas weIl as the detailed strueture of the two summits with a 10m resolution. Thecode must compute local conditions due to the modifications of the meso-scaleflow by orography. Once it has been applied on the general area in a coarseresolution mode, the same numerical code may be used to zoom on specific

CHAPTER 3. METHODS AND INSTRUMENTS 34

sumrnits with a resolution high enough to analyse the interaetions of the buildingwork with the environment:a) In the low resolution mode, the analysed area is 6x6km wide with a 100m step.The output is presented as variations of wind direction and velocity with regardto the reference point (Parana0 in horizontal planes at each chosen altitude. Theminimum alti tude above ground is 50m.b) In the high resolution mode, the output presents local variations of winddirection and velocity with regard to the results of the low resolution mode. Thestep is then 10m and the lowest altitude above ground is 5m.

The complete results of this study are expected for the end of 1990.

References[1] M. Azouit, J. Verninj Remote investigation 01 tropospheric turbulence bytwo-dimensional analysis 01 stellar scintillation; J. Atmos. Sei., 37, 1550 (1980).[2] G. Weigclt et al.j Speckle masking, speckle speetroscopy and optical aperturesynthesis Proe. Seeond Workshop on ESO's Very Large Telcseope, Veniee, 29Sep.-2 Od. 1986.[3] M. Sarazin, F. Roddier The E.S.O DijJerential Image Motion Afonitor;Astron. Astrophys. 227, 294-300 (1990).[4] D. Morse, F. Gillett; Water vapor monitor engineering reportj AURAEngineering report n073, Od. 1982, KPNO, Tueson Az.[5] L. Zagoj Environmental ejJeets and enclosure design lor large telescopes; 88,Proe. Int. Conf. on "Very Large Telescopes and their instrumentation",Garching, 21-25 Mareh 1988.[6J 1. Troen, E.L. Petersen European Wind Atlasj published for the Commissionof the European Communities by Risoe National Laboratory. 1989, 656pp.

Chapter 4

RESULTS OF THE TESTCAMPAIGN

4.1 Location and characteristics of sites under considera­tion

4.1.1 Geography

• Cerro Armazoni (or Armazones)

Location:Altitude:Distance from coast:Highest neighboring peak:Form of summit:Vegetation at summit :Light pollution:Nearest reasonable town:Nearest airport:Airline connection:Glosest mining activity:

• Cerro Paranal

Location:Altitude:Distance from coast:Highest neighboring peak:Form of summit:

70°11'21" W, -24°35'15"8.3.064 m.34 km.Sierra Vicunia Mackenna, 3078 m, 15 km E.saddle peak.none.none.Antofagasta, 140 km, 200.000 h.Gerro Moreno (Antofagasta), 160km.4 daily flights to and from Santiago.Gerro Yumbes, small copper mine at30 km to the SSW.

70°23'15" W, -24°37'00"8.2.665 m.12 km.Gerro La Ghira, 2569 m, 11 km to the NNE .peak.

35

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 36

Vegetation at summit :Light pollution:Nearest reasonable town:Nearest airport:Airline connection:Closest mining aetivity:

• Cerro La Montura

Location:Altitude:Distance from coast:Highest neighboring peak:Form of summit:Vegetation at surnmit :Light pollution:Nearest reasonable town:Nearest airport:Airline connection:Glosest mining aetivity:

• Cerro Vizcachas

Location:Altitude:Distance from coast:Highest neighboring peak:Distance from other sites:Form of summit:Vegetation at summit :Light pollution:Nearest reasonable town:Nearest airport:Airline co~neetion:

Glosest mlning activity:

none.none.Antofagasta, 130 km, 200.000 h.Gerro Moreno (Antofagasta), 150km.4 daily fiights to and from Santiago.Cerro Yumbes, small copper mine at15 km to the SSWj small coastal coppermines at 10 km to the West.

70°22'53" W, -24°36'00" S.2.516m (+2.522 m peak at 500m southwards).14 km, 3750 m NE of Paranal.Cerro La Chira, 2569 m, 11 km to the NNE .double peak.none.none.Antofagasta, 130 km, 200.000 h.Gerro Moreno (Antofagasta), 150km.4 daily fiights to and from Santiago.Gerro Yumbes, small copper mine at18 km to the SSWj small coastal coppermines at 12 km to the West.

70°44'53" W, -29°15'20"S.2.389 m (+2.398 m peak at 530m northwestwards).40 km.Gerro Puquios Blancos, 2974m, 13km ESE.6 km SE of La Silla, 800 km South of Paranal.double peak.some grass, a few low bushes.city of La Serena at 100km to the SW.La Serena-Coquimbo, 160 km, 240.000 h.La Florida (La Serena), 170km1 daily fiight to and from Santiago + ESO plane.La esperanza, several small copper mines at10 km to the South.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

• Le Grand Benare

37

Loeation:Altitude:Distanee from eoast:Highest neighboring peak:Distanee from other sites:Form of sumrnit:Vegetation at summit :Light pollution:Nearest reasonable town:Nearest airport:

55°25'45" E, -21°06'10"S2.896m15km.Piton des neiges, 3070m, 6km ENE.9500km from Europe.ridge of a erater.some grass.very limitedSaint Denis, 100 km, 100.000 h.Saint Denis, 100km

4.1.2 Overview of available data

Here follows a summary of the presently available data on sites and sub-sites:

LA SILLA •

• Cloudiness (visual): 1965 to 1972 and 1983 onward.

• Water Vapour (optieal monitor): every 2h daytime and 1h nighttime,from 1983 to January 1989 (then moved to Vizeaehas).

• Meteorological data: Vaisala Station (automatie recording), 20mn av­erages, max., min., dev. of: Wind (10m, 20m, 30m), Temperature (2m,ground), Relative humidity (2m); from 1985 onward.

• SODAR (Automatie reeording) 20mn averages of vertieal turbuleneewith 40m resolution from 50 to 500-700m : Feb. 1986-Apr. 1987.

• Seeing Monitor: Imin averages during nighttime only: March 1986,Nov. 1986, April 1987.

• Seeing statisties : 3.6m, 2.2m, Danish, telescopes in CCD direet imag­ing mode, from January 1987 onward.

VIZCACHAS •

• Water Vapour (optical monitor) and cloudiness (visual): every 2h day­time and 1h nighttime, from January 1989 onward.

• Meteorologieal data: Vaisala Station (automatie reeording: 20mn av­erages, max., min., dev. of: Wind (10m), Temperature (2m, ground),Relative humidity (2m); from Oet. 1988 onward.

• Seeing Monitor: Imin averages during nighttime only: Oet. 1988 on­ward.

• SODAR, Scintillometer from Mareh 89 onward.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 38

PARANAL •• Water Vapour (optieal monitor) and cloudiness (visual): every 2h day­

time and Ih nighttime, from 1983 onward.

• 22GHz Water Vapour monitor: automatie reeording; from Sep. 1988onward.

• Meteorologieal data: Vaisala Station (automatie rceording), 20m aver­ages, max., min., dev. of: Wind (2m, 10m), Temperature (2m, ground),Relative humidity(2m); from 1985 onward.

• SODAR (Automatie reeording) 20mn averages of vertieal turbuleneewith 40m resolution from 50 to 500-700m : Apr. 1987-Apr. 1988.

• Seintillomcter, mierothermal at 5m: Imin averages during nighttimeonly: from April 1987 to Mareh 1989.

• Seeing monitor: from April 1987 onwards.

LA MONTURA •• Meteorologieal data: Semi automatie station, Wind, Tempcrature at

2m from Oetober 1988 to January 1990.

• Meteorologieal data: Solus Station (automatie reeording), 20m avcr­ages, max., min., dev. of: Wind (6m), Temperature (2m, ground), Rel­ative humidity(2m), solar radiation; from Oet. 1989 onward.

• Seeing Monitor, seintillometer: Imin averages during nighttime only:from April 1989 to October 89.

• Mierothermal sensors: from Aug. 89 to October 89.

ARMAZONI •

• Meteorologieal data: Semi automatie station, Wind, Temperature at2m from February 1987 to July 22,1988 and from January 90 onward.

• Meteorologieal data: Vaisala Station (automatie reeording), 20m aver­ages, max., min., dev. of: Wind (10m), Temperature (ground), Pressurefrom February 1986 to Mareh 15, 1986.

• Seeing Monitor, seintillometer: Imin averages during nighttime only:from 15 Feb. to 15 Apr. 1990.

LE GRAND BENARE •

• Water Vapour (optieal monitor) and cloudiness (visual + photographie):3 times per night, two nights per week from August 1986 to May 1987(under the responsability of INSU).

• Meteorologieal data: Vaisala Station (automatie reeording), 20m aver­ages, max., min., dev. of: Wind (10m), Temperature (ground), Pressurefrom August 1986 to May 1987.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 39

4.1.3 Topographie maps

Fig. 4.1 to 4.4 are detailed maps produced by ESO/Chile Maintenance and Con­struction department. The original scale of 1/1000 has been reduced to 1/5000.

The topography of the site has a major impact on cost of infrastructure. More­over, because the Sm units require space in the East-West direction and theauxiliary interferometric telescopes need a North-South baseline, the final ar­rangement of the telescopes cannot be decided before the site is chosen.

---Figure 4.1: Map of thc summit of Armazoni (scale 1/5000)

-----t------+-~

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 40

)I %175000

,_.---

'"

i

~.~~ .'

2600 -1_-

_..-

__0' ._-+-__-

-"-"-"

- ---~~--

- -

------L-----t--- 2~50--l-__-

.--'-'

---.. -"- -

,--

II

: II ,,

, I

,/ /(1I I

I

/', /,/

,/

zo~,.,

Figure 4.2: Map of the summit of Paraual (scale 1/5000)

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 41

I/f

. N 7.279.000

I

\\

\\\

\\

\\ \

) \I '

~

3 I/

(I\

,.~- ....-------_._ .. -"'~:'~'-""'--~'-~=:':'-'\.' 0 .' ... '\1.,,0 .•.. /' .__

'-------r~-

I!

~ i J

/

8 j /~/ / 8. /~ i /

! JI I

/ /

zo~'"

<~ !. I t. I rfr-.äIco..p.

Figure 4.3: Map of the summit of La Montura (scale 1/5000)

(I

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Figllre 4.4: Map of thc sununit of Vizcachas (scale 1/5000)

~'"z

3,UION·

42

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.2 Cloud Cover

43

4.2.1 La Silla

Already when the first site survey work for the ESO VLT was initiated in North­ern Chile, in 1982, it was deeided that La Silla was to be regarded not only as astrong potential site eandidate but also as an exeellent referenee. The long anddetailed experienee of La Silla as a site and its proven high quality were' essentialfeatures. In addition, with its position in the southern part of the Chilean desertregion, it seemed rather adequate to eompare site quality parameters of La Sillawith those to be reeorded for site eandidates in Northern Chile.

In eonsequenee, when detailed site quality monitoring started on Paranal, inSeptember 1983, parallel monitoring was made at La Silla. With equal eriteriaand instrumentation, it was endeavored to obtain observing data as eomparableas possible for the two plaees. Parallel monitoring at La Silla and at Paranalhave eontinued and still do eontinue.

Regarding cloud cover, the site evaluation eampaign initiated in September1983, has until the er"d of 1989, resulted in an uninterrupted reeord including atotal of more than 2200 nights monitored at La Silla. This should ensure asolidstatistieal ba.sis for evaluations of the photometrie quality of the night sky atLa Silla. At the same time, the period included should be long enough also forjudgement of the possible existenee of long-term trends.

For La Silla, and espeeially for an evaluation of possible long-term trends, webenefit from earlier ree.ords of photometrie sky quality. Regular classifieations ofphotometrie night sky qualities were taken up already iu 1965 (Muller, 1966) andwere eontinued in thc years to follow (Muller, 1968a, b, 1969; Westerlund, 1971,1972,1974). The data on photometrie. sky quality from 1965-1972 havc been usedfor a seareh for possible trends within this time span and also for a eomparisonto and an extensio'1. of ~he data obtained during the site evaluation eampaign forthe ESO VLT.

In Table 4.1, data are provided on photometrie sky quality at La Silla forthe period 1965-1972 (Ardeberg et al., 1990a). In Figures 4.5 to 4.7 , monthlypereentages of photometrie nights are plotted versus month for the same period.Data on photometrie and speetroseopie nights at La Silla are shown in Tables 4.2and 4.3 for the period September 1983 to November 1989. Figures 4.9 to 4.11display monthly pereentages of photometrie nights, plotted versus month, for thesame period.

In order to evaluate the signifieanee of possible long-term trends in the pho­tometrie sky quality at La Silla, we have, in Figure 4.8, displayed monthly per­eentages of photometrie nights from 1965 to 1972 versus time. In Figure 4.12,eorresponding graphieal data are shown for the period 1983-1990. Finally, inFigure 4.13, average monthly pereentages of photometrie nights at La Silla areplotted versus month. The dashed curve reports data for the period 1965-1972,

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Month 1965 1966 1967 1968 1969 1970 1971 1972 1965-1972Average

January 100 94 71 58 74 61 74 76.0February 73 96 100 62 82 64 89 86 81.5

March 79 94 90 81 71 58 87 87 80.9April 33 57 73 70 50 77 73 67 62.5May 15 58 35 55 39 39 51 48 42.5June 6 47 67 38 20 50 40 23 36,4July 9 36 52 42 35 45 45 55 39.9

August 25 64 39 58 52 45 48 26 44.6September 63 70 52 37 53 37 47 43 50.3

October 63 64 58 49 45 48 58 45 53.8November 65 50 57 80 67 77 57 76 66.1December 77 94 71 90 84 90 71 65 80.3Average 46.2 69.2 65.7 61.1 54.7 58.7 60.6 57.9 59.4

44

Table 4.1: Data on photometrie sky quality at La Silla for the period 1965-1972. Forevery month, the pereentage of photometrie nights is given. Finally, monthly averagesare shown for the entire period.

whilst the full-drawn eurve shows eorresponding data for the period 1983-1990.Monthly pereentages of speetroseopie nights at La Silla are, in Figures 4.14to 4.16, shown for the period 1983-1990. Data supporting an evaluation of pos­sible long-term trends in speetroseopie sky quality at La Silla are displayed inFigure 4.17. Monthly pereentages of speetroseopie nights are given versus timefor the period September 1983 to November 1989 .

In order to improve our abilities to deteet trends with larger periods at La Silla,we have treated our observing data on photometrie and speetroseopie sky qualitywith a time filter with a width of 12 months. The results of sueh filtering areshown in Figures 4.18 to 4.20. Figure 4.18 shows the data relevant to photometriesky quality for the period 1965 to 1972, whilst Figure 4.19 gives the eorrespondingdata for the period September 1983 to November 1989. In Figure 4.20, dataregarding speetroseopie sky quality are shown for the same period as eovered inFigure 4.19.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 45

Nights Nights Nights % % Nights Nights Night % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1983 1984January 31 23 21 74 68February 29 28 23 97 79

March 31 27 21 87 68April 30 28 24 93 80May 31 20 13 65 42June 30 17 7 57 23July 31 13 11 42 35

August 31 23 15 74 48September 14 12 8 86 57 30 16 10 53 33

October 31 29 21 94 68 31 26 18 84 58November 30 25 15 83 50 30 25 23 83 77December 31 29 18 94 58 31 29 22 94 71

Month 1985 1986January 31 31 25 100 81 31 31 29 100 94February 28 28 27 100 96 28 28 23 100 82

March 31 27 24 87 77 31 30 26 97 84April 30 30 25 100 83 30 25 21 83 70May 31 27 16 87 52 31 16 8 52 26June 30 20 8 67 27 30 20 8 67 27July 31 18 10 58 32 31 19 17 61 55

August 31 22 12 71 39 31 16 7 52 23September 30 24 19 80 63 30 23 19 77 63

October 31 23 7 74 23 31 18 6 58 19November 30 27 19 90 63 30 21 10 70 33December 31 30 21 97 68 31 28 17 90 55

Table 4.2: Data on photometrie and speetroseopic sky quality at La Silla for the pe­riod 1983-1986. For every month, the numbers and pereentages of photometrie andspeetroseopie nights are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 46

Nights Nights Nights % % Nights Nights Nights % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1987 1988January 31 31 23 100 74 31 31 29 100 94February 28 25 14 89 50 29 29 22 100 76

March 31 27 16 87 52 31 30 26 97 84April 30 24 14 80 47 30 26 20 87 67May 31 17 11 55 35 31 20 11 65 35June 30 26 19 87 63 30 24 12 80 40July 31 11 7 35 23 31 23 16 74 52

August 31 20 12 65 39 31 27 9 87 29September 30 17 12 57 40 30 27 18 90 60

October 31 20 8 65 26 31 31 29 100 94November 30 28 25 93 83 30 30 24 100 80December 31 30 21 97 68 31 30 23 97 74

Month 1989 1990January 31 31 16 100 52February 28 28 18 100 64

March 31 31 30 100 97April 30 27 23 90 77May 31 24 19 77 61June 30 28 17 93 57July 31 25 15 81 48

August 31 23 18 74 58September 30 24 16 80 53

October 31 28 19 90 61November 30 26 18 87 60December

Table 4.3: Data on photometrie and spcetroseopie sky quality at La Silla for the pe­!iod 1987-1990. For every month, thc numbers and pereentages of photometrie andspeetroseopic nights are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 47

100

BO

Ez 600 \H \I- \U \ ,< 40 \ ,a: \IIJ.. .

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.5: Photometrie night sky quality at La Silla for the years 1965, dotted eurve,1966, full-drawn eurve, and 1967, dashed eurve. Monthly pereentagcs of photometrienights are plotted versus month.

100

80•, , \

E," .\, '\

\

60\.

Z\',\

0 \H \ ~" ..I- \ .~U \ ~.

< 40~

a: ~

IJ..

20

0J F M A M J JAS 0 N 0

MONTH

Figure 4.6: Same as Figure 4.5 but for the years 1968, dotted curve, 1969, full-drawneurve, and 1970, dashed curve.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

Ez 600I-l...(J« 40a:u.

20

0J F M A M J J A S 0 N 0

MONTH

48

Figurc 4.7: Same a.s Figure 4.5 hut far the years 1971, dotted eurve, and 1972, full-drawn

eurvc.

zoI-l...(J«a:u.

1965 1966 1967 1968 1969 1970 1971 1972YEAR

Figure 4.8: Manthly pereentages af photometrie nights at La Silla plotted versus timefar the period 1965-1972.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 49

Figure 4.9: Same as Figure 4.5 hut for the years 1983. dutted curvc, 1984. full-drawncurve, and 1985, dashed curve.

100

"I ,I ,

I ,80 I

,I

II

EI

I0/

60 ',.z ' .0

,H

,.... ,u ,< 40

,a:u.

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.10: Same as Figure 4.5 hut for the years 1986, dotted curve, 1987, full-drawncurve, and 1988, dashed curve.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

~

z 600H....U< 40a:11.

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.11: Same as Figure 4.5 hut for the year 1989.

100

80

Ez 600H....U< 40a:11.

20

01883 1884 1885 1886 1887 1888 1888

YEAR

Figure 4.12: Same as Figure 4.8 hut for the period 1983-1990.

50

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 51

100

80

60zoH.­(J

~ 40u.

20

,...., ,~---~ ....,,,,,,,,

o N 0 J F M A M J JAS 0 N 0 J F MMONTH

Figure 4.13: Test for a possible long-term trend in photometrie sky quality at LaSilla. We eompare average monthly perecntages of photometrie llights for thc periods1965-1972, indicated by a dashed curve, and 1983-1990, indicated by a full-urawn curvc.Both eurves eover an interval of 18 months.

100

BO

E60z

0H.-(J

< 40Cl:u.

20

0J F M A M J J A S 0 N 0

MONTH

Figurc 4.14: Spcetroseopie night sky quality at La Silla for thc ycars 1983, dottcd curve,1984, full-drawn eurve, and 1985, dashcd curve. Monthly percentages of spectroscopicnights are plotted versus month.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

--~60z

0totI-UCl: 40a:u.

20

0J F M A M J J A S 0 N 0

MONTH

52

Figure 4.15: Same as Figure 4.14 hut for the years 1986, dotted curve, 1987, full-drawncurve, and 1988, dashed curvc.

100

80

Ez 600totI-UCl: 40a:u.

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.16: Same as Figure 4.14 hut for the year 1989.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 53

100

80

Ez 600Ht-Uoe( 40a:u.

20

01983 1984 1985 1986 1987 1988 1989

YEAR

Figurc 4.17: Monthly perecntages of spectroseopie nights at La Silla plotted vcrsustime for thc period 1983-1990.

1968 1969 1970 1871 1972YEAR

0L--....L----l_.J--L_.J..----L.._1.--J----l_....L...........L._-'----L._~

1965 1966 1967

100

80

,..~

60z0I-lt-Uoe( 40a:u.

20

Figure 4.18: Test for possible long-term trends in photometrie sky quality at La Silla.Monthly pereentages of photometrie nights are given from 1965 to 1972, passed througha time filter having a width of 12 months.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

--~60z

0t-4I-Uer: 40a:lI..

20

01984 1985 1986 1987 1988 1989

VEAR

Figure 4.19: Same as Figure 4.18 but for the period 1983-1990.

100

80

E60z

0t-4I-Uer: 40a:lI..

20

01984 1985 1986 1987 1988 1989

VEAR

54

Figure 4.20: Test fpr possible long-term trends in spectroscopic sky quality at La Silla.Monthly percentages of spectroscopic nights are shown from 1983 to 1990, passedthrough a time filter having a width of 12 months.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.2.2 Paranal

55

Detailed monitoring of the photometrie sky quality at the summit of Paranal hasbeen an important part of the site evaluation programme for Northern Chile. Thec10ud eover data obtained at Paranal are of high interest not only for evaluatingthe site quality of Paranal proper. In eombination with the eorresponding dataobtained at La Silla, the c10ud eover observations from Paranal serve as a eom­parison between the La Silla and Paranal areas. Further, the observing data onc10ud eover at Paranal are regarded as a standard referenee for eorrespondingobservations earried out at other mountain summits in Northern Chile.

As for La Silla, the number of nights for whieh c10ud eover observations havebeen made until the end of 1989, is more than 2200. Again as for La Silla, allnights between September 1983, and November 1989, are inc1uded and observa­tions are still eontinued. In addition, at Paranal also the daytime sky has beeneonstantly monitored during the same time. Given the importanee for the VLTof observations at infrared wavelengths, the photometrie quality of the daytimesky is of high importanee. Finally, the inc1usion of daytime observations providespossibilities for a study of the diurnal c10ud eover eyc1e and its stability andrelation to other site parameters.

With the extremely high photometrie sky quality prevailing in the Paranalarea, thc observing series obtained attains a partieularly high weight. This isemphasized by the uneommonly small seasonal variations. As a result, the c10udeover data aeeumulated for Paranal form a solid basis for eonc1usions eoneerningsite quality, inc1uding a seareh for possible long-term trends.

For the period September 1983 to November 1989, Tables 4.5 and 4.6 give dataon photometrie and speetroseopie night sky quality at Paranal. In Tables 4.7, 4.8,4.9 and 4.10, eorresponding data on photometrie and speetroseopic sky qualityat Paranal are shown for daytime and morning and evening hours, respeetively.

Monthly pereentages of photometrie nights at Paranal, displayed versus month,are shown in Figures 4.21 to 4.23 , eovering the same period as Tables 4.5 and 4.6.For a study of possible long-term trends, we give, in Figure 4.24, monthly per­eentages of photometrie nights for the entire period 1983-1990, plotted versustime.

For speetroseopie nights at Paranal, over the period September 1983 to Novem­ber 1989, we show in Figures 4.25 to 4.27, the statistics corresponding to thosegiven for photometrie nights in Figures 4.21 to 4.23. Similarly, Figurc 4.28 re­ports, for speetroseopie nights, data eorresponding to those displayed for photo­metrie nights in Figure 4.24.

As for the data from La Silla, we have treated our data from Paranal onphotometrie and speetroseopic night sky quality with a time filter with a widthof 12 months. The resulting data are shown in Figures 4.29 and 4.30, givingresults for photometrie and speetroseopie nights, respectively.

It is noted, that the eoneept of photometrie nights (and days) employed in our

ClIAPTER 4. RESULTS OF THE TEST CAMPAIGN 56

Nights Nights Nights % % Nights Nights Night % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1986 1987January 15 10 4 67 27February 5 2 0 40 0

March 4 2 2 50 50April 4 3 2 75 50May 5 4 3. 80 60JuneJuly

August 6 5 3 83 50September 9 9 8 100 89

October 9 7 2 78 22November 8 7 3 88 38December 13 5 3 38 23

Table 4.4: Data on photometrie and spectroseopie sky quality at Le Grand Denare forthe period 1986-1987. For every month, the numbers and pereentages of photometrieand speetroseopic nights are given.

studies of sky quality is a rather striet way of judging the photometrie qualityof the sky. An alternative and less demanding eoneept is that of aeeumulatedphotometrie hours. The two eoneepts have been eompared in praetiee for datafrom La Silla as weH as from Paranal (Ardeberg et al. , 1990a). For La Silla,the eoneept of aeeumulated photometrie hours results in an average number ofphotometrie hours per night of 5.3, whilst the eoneept of photometrie nights givesa eorresponding number of 4.8. For Paranal, equivalent eoneepts result in 7.6 and7.3 hours per night, respeetively. The small differenees in the results based onthe two eoneepts testify to the exeellent quality of the night sky at La ·Silla andat Paranal.

4.2.3 Grand Benare

Our detailed monitoring of the photometrie quality of the sky above GrandBenare included a total of 78 nights, distributed over 10 months between August1986 and May 1987 (Table 4.4). Out of those nights, 30 were of photometrieand 54 of speetroseopic quality, eorresponding to 38% and 69%, respeetively.Evidently, during the period studied, the photometrie quality of the sky aboveGrand Benare was signifieantly lower than those presented at the Chilean sitesunder study.

References: se(;1: Chapter 2

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 57

Nights Nights Nights % % Nights Nights Night % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1983 1984January 31 22 19 71 61February 29 25 22 86 76

March 31 27 25 87 81April 30 29 27 97 90May 31 30 26 97 84June 30 23 23 77 77July 31 24 24 77 77

August 31 28 25 90 81September 14 14 13 100 93 30 27 27 90 90

October 31 31 29 100 94 31 19 17 61 55November 30 30 30 100 100 30 30 30 100 100December 31 30 30 97 97 31 29 28 94 90

Month 1985 1986January 31 31 29 100 94 31 27 24 87 77February 28 25 23 89 82 28 28 28 100 100

March 31 31 26 100 84 31 30 28 97 90April 30 30 28 100 93 30 29 27 97 90May 31 28 24 90 77 31 27 22 87 71June 30 24 19 80 63 30 27 21 90 70July 31 29 27 94 87 31 27 26 87 84

August 31 26 25 84 81 31 24 22 77 71September 30 30 30 100 100 30 30 28 100 93

October 31 29 27 94 87 31 29 24 94 77November 30 27 20 90 67 30 25 21 83 70December 31 28 23 90 74 31 29 21 94 68

Table 4.5: Data on photometrie and spectroscopie sky quality at Paranal for the pe­riod 1983-1986. For every month, the numbers and pereentages of photometrie andspeetroseopic nights are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 58

Nights Nights Nights % % Nights Nights Night % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1987 1988January 31 26 25 84 81 31 31 29 100 94February 28 20 17 71 61 29 29 27 100 93

March 31 26 25 84 81 31 29 24 94 77April 30 29 24 97 80 30 25 22 83 73May 31 25 22 81 71 31 26 22 84 71June 30 27 22 90 73 30 28 21 93 70July 31 24 21 77 68 31 29 25 94 81

August 31 23 22 74 71 31 29 22 94 71September 30 25 22 83 73 30 28 25 93 83

October 31 27 25 87 81 31 31 29 100 94November 30 29 28 97 93 30 30 30 100 100December 31 30 28 97 90 31 30 22 97 71

Month 1989 1990January 31 30 24 97 77February 28 28 27 100 96

March 31 31 30 100 97April 30 28 27 93 90May 31 28 23 90 74June 30 22 21 73 70July 31 28 24 90 77

August 31 30 29 97 94September 30 26 25 87 83

October 31 25 23 81 74November 30 27 25 90 83December

Table 4.6: Data on photometrie and speetroseopie sky quality at Paranal for the pe­riod 1987-1990. For every month, the numbers and pereentages of photometrie andspeetroseopie nights are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 59

Days Days Days % % Days Days Days % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1983 1984January 31 25 19 81 61February 29 26 20 90 69

March 31 27 14 87 45April 30 29 26 97 87May 31 24 17 77 55June 30 20 9 67 30July 31 26 19 84 61

August' 31 25 19 81 61September 13 13 12 100 92 30 26 19 87 63

October 31 30 28 97 90 31 18 13 58 42November 30 30 27 100 90 30 26 19 87 63December 31 29 28 94 90 31 27 16 87 52

Month 1985 1986January 31 30 22 97 71 31 27 18 87 58February 28 26 16 87 57 28 27 21 96 75

March 31 28 20 90 65 31 29 22 94 71April 30 28 24 93 80 30 24 22 80 73May 31 22 16 71 52 31 24 17 77 55June 30 16 13 53 43 30 25 19 83 63July 31 25 21 81 68 31 25 23 81 74

August 31 26 25 84 81 31 24 17 77 55September 30 29 29 97 97 30 28 25 93 83

October 31 28 22 90 71 31 24 20 77 65November 30 22 11 73 37 30 21 21 70 70December 31 27 19 87 61 31 21 9 68 29

Table 4.7: Data on photometrie and speetroseopie sky quality at Paranal for the pe­riod 1983-1986. For every month, the numbers and pereentages of photometrie andspectroseopie days are given.

CIIAPTER 4. RESULTS OF TIIE TEST CAMPAIGN 60

Days Days Days % % Days Days Days % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1987 1988January 31 26 14 84 45 31 24 17 77 55February 28 20 11 71 39 29 26 20 90 69

March 31 25 17 81 55 31 25 18 81 58April 30 23 19 77 63 30 26 12 87 40May 31 23 14 74 45 31 21 12 68 39June 30 23 17 77 57 30 17 15 57 50July 31 21 15 68 48 31 26 18 84 58

August 31 21 18 68 58 31 25 21 81 68September 30 24 22 80 73 30 26 22 87 73

October 31 26 20 84 65 31 29 28 94 90November 30 28 25 93 83 30 30 30 100 100December 31 29 26 94 84 31 28 17 90 55

Month 1989 1990January 31 31 28 100 90February 28 24 18 86 64

March 31 28 26 90 84April 30 28 23 93 77May 31 24 20 77 65June 30 21 18 70 60July 31 24 22 77 71

August 31 27 24 87 77September 30 27 21 90 70

October 31 25 22 81 71November 30 27 25 90 83December

Table 4.8: Data on photometrie and speetroseopie sky quality at Paranal for the pe­riod 1987-1990. For every month, the numbers and pereentages of photometrie andspectroseopie days are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 61

Morning/Evening % % Morning/Evening % %monit. spectr. phot. spectr. phot. monit. spectr. phot.. spectr. phot.

Month 1983 1984January 62 42 39 68 63February 58 44 40 76 69

March 62 53 41 85 66April 60 56 50 93 83May 62 47 33 76 53June 60 36 28 60 47July 62 48 40 77 65

August 62 52 36 84 58September 27 27 25 100 93 60 52 46 87 77

October 62 61 57 98 92 62 33 25 53 40November 60 59 58 98 97 60 53 44 88 73December 62 59 49 95 79 62 53 39 85 63

Month 1985 1986January 62 57 48 92 77 62 53 43 85 69February 56 48 36 86 64 56 53 48 95 86

March 62 50 41 81 66 62 59 51 95 82April 60 55 45 92 75 60 51 40 85 67May 62 46 38 74 61 62 39 34 63 55June 60 37 26 62 43 60 48 41 80 68July 62 49 40 79 65 62 45 44 73 71

August 62 44 33 71 53 62 44 36 71 58September 60 59 57 98 95 60 57 49 95 82

October 62 52 39 84 63 62 48 39 77 63November 60 43 23 72 38 60 46 37 77 62December 62 51 41 82 66 62 42 23 68 37

Table 4.9: Data on photometrie and spectroseopic sky quality at Paranal for the pe­riod 1983-1986. For every month, the numbers and pereentages of photometrie andspeetroseopie mornings and evenings, treated together, are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 62

Morning/Evening % % Morning/Evening % %monit. spectr. phot. spectr. phot. monit. spectr. phot. spectr. phot.

Month 1987 1988January 62 54 34 87 55 62 53 37 85 60February 56 37 31 66 55 58 54 46 93 79

March 62 47 41 76 66 62 50 36 81 58April 60 48 41 80 68 60 42 27 70 45May 62 44 33 71 53 62 39 28 63 45June 62 44 33 71 53 60 35 31 58 52July 62 44 32 71 52 62 50 39 81 63

August 62 40 39 65 63 62 50 39 81 63September 60 49 42 82 70 60 46 40 77 67

October 62 50 46 81 74 62 59 56 95 90November 60 54 51 90 85 60 60 59 100 98December 62 57 51 92 82 62 54 36 87 58

Month 1989 1990January 62 58 50 94 81February 56 47 41 84 73

March 62 57 55 92 89April 60 50 49 83 82May 62 46 38 74 61June 60 37 34 62 57July 62 49 42 79 68

August 62 59 53 95 85September 60 47 43 78 72

October 62 46 41 74 66November 60 51 48 85 80December

Table 4.10: Data on photometrie and speetroseopie sky quality at Paranal for theperiod 1987-1990. For every month, the numbers and pereentages of photometrie andspeetroseopie mornings and evenings, treated together, are given.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

Ez 600t-4I-U< 40a:u.

20.

0J F M A M J J A S 0 N 0

MONTH

63

Figure 4.21: Photometrie night sky quality at Paranal for the years 1983, dotted curve,1984, full-drawn curve, and 1985, dashed eurve.

zot-4I­U<a:u.

100

80

60

40

20

,'\"' ....

" ,. ."

: ,,'.. '",. .,',. ;'

' ..

J F M A M J JAS 0 N 0MONTH

Figure 4.22: Same as Figure 4.21 hut for the years 1986, dotted eurve, 1987, full-drawneurve, and 1988, dashed curve.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

E60z

0I-l...tJ< 40a:IJ..

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.23: Same as Figurc 4.21 hut for thc year 1989.

100

80

....!!

60z0I-l...tJ< 40a:lL

20

01883 1884 1885 1886 1887 1888 1888

YEAR

64

I

Figure 4.24: Morithly pereentages of photometrie nights .at Paranal plotted versus timefor the period 1983-1990.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 65

100 f I j I, ,,----,,, ,, ,, ,,

80

Ez 6001-1I-CJoe( 40a:LL

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.25: Spectroscopic night sky quality at Paranal for the years 1983, dotted curve,1984, full-drawn curve, and 1985, dashed curve. Monthly percentages of spectroscopicnights are plotted versus the month of the year.

100

80

E60z

01-1I-CJoe( 40a:LL

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.26: Same as Figure 4.25 hut for the ycars 1986, dotted curve, 1987, full-drawncurve, and 1988, dashed curve.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

100

80

,....~

z 600H~U< 40CI:u.

20

0J F M A M J J A S 0 N 0

MONTH

Figure 4.27: Same as Figure 4.25 hut for the year 1989.

100

80

E60z

0H~U< 40CI:u.

20

01983 1984 1985 1986 1987 1988 1989

YEAR

66

Figure 4.28: Mon~hly percentages of spectroscopic nights at Paranal plotted versustime for the period 1983-1990.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 67

100

80

E60z

0I-l...(Jc( 40a:11.

20

01984 1985 1986 1987 1988 1989

YEAR

Figure 4.29: Test for possible long-term trends in photometrie sky quality at Paranal.Monthly pereentages ofphotometrie nights are given from 1983 to 1990, passed througha time filter having a width of 12 months.

1988 1989

~.

100

80

Ez 600I-l...(Jc( 40a:11.

20

01984 1985 1986 1987

YEAR

Figure 4.30: Test for possible long-term trends in speetroseopic sky quality at Paranal.Monthly pereentages of speetroseopie nights are plotted from 1983 to 1990, passedthrough a time filter having a width of 12 months.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.3 Water Vapour in the Atmosphere

4.3.1 La Silla area

68

Parallel to evaluations of photometrie sky quality, measurements have been madeof the integrated eontent of water vapour of the atmosphere above La Silla. Allour data on atmospherie water vapour were obtained with the sky radianee mon­itors detailed above. Measurements have been made in an open plaee relativelyelose to the Sehmidt teleseope. It has been attempted to inelude both nighttimeand daytime with a frequency as high as possible. Unfortunately, various cir­eumstances have made it impossible to obtain observing frequencies similar tothose for Paranal. In addition, it was not possible to maintain regular observingintervals.

For the period for whieh reduced data on integrated eontent of water vapourin the atmosphere above La Silla exist, from September 1983 until the end of1989, our results are presented in Tables 4.11 and 4.12. Table 4.11 presents dataobtained during nighttime, whilst Table 4.12 shows eorresponding data obtainedduring daytime. The results of our measurements of integrated atmospheric watervapour above La Silla are shown graphieally in Figures 4.31 to 4.38.

Figures 4.31 to 4.33 show the distribution of water vapour content expressed interms of the amount of precipitable water vapour for nights during winter time,spring and fall time, and summer time, respeetively. We have defined winter timeas from June 1 to September 30, summer time as from December 1 to March 31,and spring and fall time as the part of the year remaining. Figures 4.34 to 4.36give the same distributions as shown in Figures 4.31 to 4.33 but for daytime.

The seasonal cycle of water vapour content of the La Silla atmosphere is demon­strated in Figure 4.37 as referred to data obtained during nighttime. The amountof preeipitable integrated atmospherie water vapour above La Silla is plotted vcr­sus observing epoeh for the total period studied. Corresponding data referringto daytime are displayed in Figure 4.38.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Period First quartile Median Last quartile(month and day) (mmH20 ) (mmH20 ) (mmH20 )

0101-0120 5.92 6.96 8.130121-0210 6.68 7.43 8.28

0211-0228 4.45 5.74 6.14

0301-0320 4.21 5.64 7.45

0321-0410 3.25 4.74 6.780411-0430 2.35 3.22 3.790501-0520 2.00 2.42 3.20

0521-0610 1.08 1.84 3.67

0611-0630 3.45 3.94 4.13

0701-0720 1.33 1.34 1.38

0721-0810 1.32 1.67 1.86

0811-0831 1.75 3.30 3.880901-0920 1.33 1.90 2.45

0921-1010 - 1.34 -1011-1031 2.66 3.52 4.45

1101-1120 2.74 4.29 5.66

1121-1210 2.93 4.11 7.77

1211-1231 4.91 6.20 7.28

69

Table 4.11: Integrated atmospheric water vapour above La Silla during nighttime. Dataare given for periods of 20 days and refer to the period 1983-1989. For each twenty­day period, data are shown for the median as well as for the two quartile values, allexpressed in terms of millimetres of precipitable water.

Period First quartile Median Last quartile(month and day) (mmH2O) (mmH2O) (mmH20 )

0101-0120 6.94 8.40 9.12

0121-0210 5.78 7.45 8.99

0211-0228 4.80 5.53 8.60

0301-0320 3.39 4.35 4.94

0321-0410 5.53 6.14 6.65

0411-0430 2.44 3.20 5.10

0501-0520 2.03 2.56 2.95

0521-0610 2.13 3.03 3.66

0611-0630 2.37 2.94 3.54

0701-0720 1.41 1.55 1.66

0721-0810 1.47 1.89 2.06

0811-0831 1.72 1.96 2.09

0901-0920 1.50 3.03 4.55

0921-1010 - 1.59 -1011-1031 3.45 4.10 4.61

1101-1120 3.16 3.20 5.10

1121-1210 4.64 6.78 7.62

1211-1231 4.95 6.26 7.50

Table 4.12: Same as Table 4.11 but for daytime data.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 70

o 2 4 6 6 10WATER VAPOUR CONTENT (mm)

12

Figure 4.31: Distribution of integrated water vapour content in the atmosphere aboveLa Silla during winter nights in the period 1983-1989. Winter time is defined as lastingfrom June 1 to September 30, both limits inclusive. The distribution, in percentages,is given for thc amount of precipitable water, expressed in millimetres.

o 2 4 6 9 10 12WATER VAPOUR CONTENT (mm)

Figure 4.32: Same las Figure 4.31 but for nights during spring and fall. Spring and falltimes are defined h.s from October 1 to November 30 and from April 1 to May 31,respectively, all limits inclusive.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

eo

..0

M~30

zoH

~ 20

10

71

o 2 .. B B 10HATER VAPOUR CONTENT (mm)

12

Figure 4.33: Same as Figure 4.31 but for summer nights. Summer time is defined asfrom December 1 to March 31, both limits inclusive.

50

..0

10

O-'--+---r-"'"""T--y--r----r-.,.---II--r--r-"'"""T--y--r----r---L..o 2 .. B B 10

HATER VAPOUR CONTENT (mm)12

Figure 4.34: Same as Figure 4.31 but for daytime.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 72

o 2 4 6 6 10WATER VAPOUR CONTENT (mm)

12

Figure 4.35: Same as Figure 4.32 but für daytime.

50

40

10

O....L-.....--1----,--.-~__r-_r_-r___,_-_r____,,.___+_-......__...---L..

o 2 4 6 6 10WATER VAPOUR CONTENT (mm)

12

\ Figure 4.36: Same as Figure 4.33 but für daytime.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 73

12

10

.'I

"

.

J ''; \ •.'0'. '., ..• ,. 0" ,

o. • .~., : •• • 0°

... f1 . :.,., •0, .1 • I',. .

..• • I.• c· •

': ., :

."

...... . 0'

.' I:\ : 0,'., .. ~ .' ~

: t·~ ...", '.~ : ~?..., .,.

2 : o' • ;' .~:-.. ~ ..',' :.\~ ... ,.

O':-"'--::-":-........-:-'::-'---=-":--''----:-L.....J.-:-L:--L--.L.......l.----l_LJ0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

FRACTION OF YEAR

Figure 4.37: Seasonal variation of the content of water vapour in the atmosphere abovcLa Silla as observed during nighttime in the period 1983-1989. Observed amounts ofprecipitable water vapour, expressed in millimetres, are plotted versus observing cpoch.The graph covers a time interval of 18 months. Plot crowding implies that zones ofhigh data density get underrepresented in such a diagram.

12

10

'.

":.::0.' ·t··I' ,

2 k;·, . .":• : '.~' : I.

'I ':

I • ~. 0:" .

h " 't'O,'. . .: r,. I :'

~ ,.'\ At I ••1.. .•• s.:-~ i . I • ~.

!: I ~ : ~: ..... • l• S '" .... • \'

\;.~",' I~ .:••!o? .. •MI'~':'~~'...

'.

t3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

FRACTI ON OF YEAR

Figure 4.38: Same as Figure 4.37 but for daytime.

ClIAPTER 4. RESULTS OF TlIE TEST CAMPAIGN 74

4.3.2 Paranal area

From the beginning of September 1983, measurements of integrated atmospheriewater vapour above Paranal have been attempted every two hours, night and day(Ardeberg et al., 1985, 1986a, b, 1987, 1990b). Only oeeasionally invalidated bypresenee of elouds and, rarely, for teehnieal reasons, the total number of reliabJemeasurements is, by the end of 1989, around 23000 for Paranal. This ensures asolid data base for eonelusions regarding average amounts of atmospherie watervapour as well as seasonal variations and variations over longer periods.

All measurements of integrated atmospherie water vapour above Paranal havebeen made fully parallel to evaluations of photometrie sky quality and generalmeteorologieal parameters. As at La Silla, all water vapour measurements weremade with the sky radianee monitors deseribed in detail above. Measurementswere made in an open plaee elose to the summit of Paranal and with windprotection required for proper functioning of the ehopping system yet ratherexposed to wind flushing.

As our water vapour measurements have been obtained with a total of three skyradianee monitors, we have made frequent eontrols of the eompatibility of the re­sults derived from observing data delivered by the different monitors. These eon­trols inelude a very large number of fully simultaneous measurement series, ob­tained with two, sometimes three, monitors mounted elose together. The overallresults of sueh intereomparisons ean be demonstrated as in Figures 4.40 to 4.43,showing eomparisons between data, expressed in terms of sky emissivity, ob­tained with the three different monitors used for our measurements of integratedatmospherie water vapour above Paranal and La Silla.

As for the eorresponding data for La Silla, our results eoneerning integratedeontent of water vapour in the atmosphere above Paranal (Ardeberg et al., 1985,1986a, b, 1987, 1990b) are presented in the form of tables. Tables 4.13 and 4.14show, respectively, the atmospherie water vapour data measured during night­time and daytime. As before, these data ean also be presented in graphieal form.Figures 4.44 to 4.51 present the data for Paranal eorresponding to those for LaSilla shown in Figures 4.31 to 4.38 . For nights during winter time, Figure 4.44displays the amount of preeipitable integrated water vapour above Paranal. InFigures 4.45 to 4.46, eorresponding data distributions are shown for nights duringspring and fall time and during summer time, respeetively. The same distributionsas given in Figures 4.44 to 4.46, but valid for daytime, are shown in Figures 4.47to 4.49.

The large amounts of measurements of integrated water vapour in the atmo­sphere above Paranal allow a elose study of the seasonal eyele of preeipitableatmospherie water vapour at this summit. Data obtained during nighttime areshown in Figure 1.50. As for La Silla, the amount of preeipitable integrated at­mospherie water {rapour above Paranal is plotted versus observing epoeh for thetotal period studied. Figure 4.51 shows the day time data eorresponding to those

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

given for nighttime in Figure 4.50.

4.3.3 Grand Benare

75

Measurements of the amount of water vapour in the atmosphere above GrandBenare. were made during a total of 78 nights, covering 10 months from August1986 to May 1987.

Whilst the first two months, August and September, gave distributions of pre­cipitable atmospheric water vapour which were of very high quality, Oetober andNovember contributed data of less impressive quality. In the months following,the content of water vapour was very high.

In total, our data on integrated water vapour in the atmosphere above GrandBenare indicate a distribution, expressed in millimetres of water vapour, of be­tween elose to 0 and more than 13, with a median value around 5 millimetre.Further, as shown on Fig. 4.39, it seems that median values may vary from be­low 2 to more than 10 millimetre, depending on the season. These data point toa total performance less favourable than that experienced at the Chilean sitesstudied in detail.

LA REUNION ALL DATA

50

40

Ezg 30I-U«a:u.. 20

10

0

0 2 4 B B 10WATER VAPOUR CONTENT (mml

12

Figure 4.39: Content of water vapour in the atmosphere above Grand Denare as ob­served during nighttime in the period Aug. 1986- May 1987

References: see Chapter 2

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Period First quartile Median Last quartile(month and day) (mmH20 ) (mmH2O) (mmH2O)

0101-0110 2.28 5.06 6.900111-0120 2.23 3.41 5.070121-0131 3.47 5.39 7.310201-0210 4.65 6.45 8.650211-0220 2.60 4.14 6.960221-0228 2.83 4.25 6.920301-0310 3.36 5.18 8.380311-0320 2.51 5.48 7.210321-0331 1.70 3.09 5.290401-0410 2.06 2.99 4.790411-0420 1.60 2.23 3.440421-0430 1.80 2.48 3.570501-0510 1.46 2.02 2.680511-0520 1.43 1.97 3.580521-0531 1.54 2.34 3.680601-0610 1.40 2.02 2.740611-0620 1.30 1.96 3.100621-0630 1.63 2.28 3.610701-0710 1.12 1.77 3.340711-0720 1.43 2.22 3.590721-0731 1.21 1.46 2.010801-0810 .99 1.49 2.010811-0820 1.07 2.03 3.130821-0831 1.25 1.98 2.860901-0910 .89 1.22 1.750911-0920 1.13 1.71 2.360921-0930 .89 1.32 1.861001-1010 1.36 1.93 3.251011-1020 1.46 2.33 3.171021-1031 1.42 1.91 2.561101-1110 1.67 1.94 2.431111-1120 1.36 1.95 3.551121-1130 1.68 2.41 4.111201-1210 2.34 3.28 4.251211-1220 2.03 2.56 3.761221-1231 2.23 3.56 6.08

76

Table 4.13: Integrated atmospheric water vapour above Paranal during nighttime. Dataare given for periods of 10 days and refer to the period 1983-1989. For each ten­day period, data are shown for the median as weH as for the two quartile values, allexpressed in terms of millimetres of precipitable water.

\I

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Period First quartile Median Last quartile(month and day) (mmH2O) (mmH2O) (mmH2O)

0101-0110 2.72 4.40 5.860111-0120 2.59 3.22 4.970121-0131 3.66 4.87 7.070201-0210 4.51 5.78 8.020211-0220 2.87 4.69 . 7.030221-0228 2.75 3.80 5.840301-0310 3.32 4.74 8.790311-0320 2.60 3.99 5.600321-0331 2.15 3.12 4.610401-0410 2.24 2.94 4.200411-0420 1.95 2.49 3.690421-0430 2.26 2.94 3.600501-0510 1.63 2.17 2.68

0511-0520 1.51 2.34 3.240521-0531 1.59 2.30 3.330601-0610 1.58 2.27 3.050611-0620 1.56 1.89 2.720621-0630 1.77 2.47 3.910701-0710 1.42 2.07 3.280711-0720 1.45 2.22 3.600721-0731 1.22 1.53 2.320801-0810 1.14 1.45 1.820811-0820 1.27 2.18 3.260821-0831 1.39 1.97 2.59

0901-0910 1.14 1.38 1.760911-0920 1.30 1.83 2.61

0921-0930 .92 1.34 1.96

1001-1010 1.37 1.53 2.32

1011-1020 1.79 2.39 3.27

1021-1031 1.70 2.09 2.68

1101-1110 1.65 2.01 2.32

1111-1120 1.58 2.22 3.26

1121-1130 1.90 2.51 3.35

1201-1210 2.48 3.18 3.83

1211-1220 2.14 2.67 3.68

1221-1231 2.22 3.68 5.09

Table 4.14: Same as Table 4.11 but for daytime data.

77

CIIAPTER 4. RESULTS OF TIIE TEST CAMPAIGN

S.O...--..---,r--"-'---Y--"-""T--,--...,...--r-,

78

0.4

0.8

Lo.....c: 0.8g

E~li...15

~

0.2

..

.""-t"...,:,

'4~'

r'• f •~ .'..

••••..

0.2 0.4 0.8 0.8 S.OATM. EMISSIVITY (Monitor sI

Figure 4.40: Comparison of sky radiance data as obtained from two sky radiance mon­itors, Nos. 1 and 2, from measurements at 20.0 Jlm. The data from monitor No. 1 havebeen plotted versus corresponding data from monitor No. 2, all expressed in terms ofsky emissivity.

0.8

NL0.....

0.8c:gr:... ••>... ·4 •InIn 0.4...15i.~~

0.2

0.2 0.4 0.8 0.8 S.O

ATM. EMISSIVITY (Monitor sIi

Figure 4.41: Same as Figure 4.40 but with measurements at 26.7 Jlm.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

S.O

#,:-0.8 .'1"

Pi ~ , ...I. .L

/t!0....c 0.8~

~H>HUl tIUl 0.4H

ffii ."'l- ~..

.,~ ·c\ ...

0.2 .-•,.

I

0.2 0.4 0.8 0.8 1.0ATM. EHISSIVITY !Monitor S)

79

Figure 4.42: Same as Figure 4.40 hut comparison of data ohtained from sky radiancemonitors Nos. 1 and 3.

1.0 r---,r--,----y--,---r--,--,...-....-,--m

0.8

PiL0....c 0.8g~H> ,.H •UlUl 0.4H

ffi •i •..c

0.2

0.2 0.4 0.8 0.8 S.O

ATM. EHISSIVITY !Monitor S)

Figure 4.43: Same as Figure 4.42 hut with measurements at 26.7 pm.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 80

o 2 01 B B 10 12WATER VAPOUR CONTENT (mm)

Figure 4.44: Distribution of integrated water vapour eontent in the atmosphere aboveParanal during winter nights in the period 1983-1989. Winter time is defined as lastingfrom June 1 to September 30, both limits inc1usive. The distribution, in pereentages,is given for the amount of precipitable water, expressed in millimetres.

o 2 01 B B 10 12WATER VAPOUR CONTENT (mm)

Figure 4.45: Sa~e as Figure 4.44 but for nights during spring and fall. Spring and falltimes are defined as from Oetober 1 to November 30 and from April 1 to May 31,respectively, all limits inc1usive.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 81

50

40

sc~30

zot-4t-U

~ 20

10

o 2 4 6 6 10 12MATER VAPOUR CONTENT (mm)

Figure 4.46: Same as Figure 4.44 but for summer nights. Summer time is defined asfrom December 1 to March 31, both limits inclusive.

124 6 8 10MATER VAPOUR CONTENT (mm)

2o

40

O...L-4----,r---r----,r---r--r--r=t=-..,...-r---,--.---,--,--J..

50

10

Figure 4.47: Same as Figure 4.44 but for daytime.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 82

122o 4 6 B 10WATER VAPOUR CONTENT ·(mml

Figure 4.48: Same as Figure 4.45 hut far daytime.

122o 4 6 B 10WATER VAPOUR CONTENT (mm)

Figure 4.49: Same as Figurc 4.46 hut far daytime.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 83

1.5 1.7

2

o L.--L--'---L.--l._'--.L--J-...~---I.---'_.l--L--'--'--L-'

0.3 0.5 0.7 0.9 1.1 1.3

FRACTION OF Y~AR

12

10

Figure 4.50: Seasonal variation of the content of water vapour in the atmosphcre aboveParanal as observed during nighttime in the period 1983-1989. Observed amounts ofprecipHable water vapour, expressed in millimetres, are plotted versus observing epoch.The graph covers a time interval of 18 months. Plot crowding implies that zones ofhigh data densHy get underrepresented in such a diagram.

12

10

!

I 8 .

i>

~..31:

2

00.3 0.9 1.1 1.3 1.5 1.7

FRACTION OF YEAR

Figure 4.51: Same as Figure 4.50 but for daytime.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.4 Meteorology

84

4.4.1 Summary of meteorological data

A eomplete analysis of meteorologieal eonditions at La Silla and Paranal is avail­able from [1] and [2], from whieh most of the following has been extraeted, inpartieular the data found in the quick referenee tables 4.15 and 4.16. The resultsof 10 months of monitoring at La Reunion are given in table 4.17.

4.4.2 Wind

• La Silla areaAn automatie Vaisala station is operated on La Silla sinee Getober 84, andon Vizeaehas sinee November 88.

Table 4.18 gives the wind veloeity distributions measured during the year1986 at La Silla and during the period November 88-June 1989 at Vizeaehas.

The eorresponding noeturnal wind veloeity distribution as a funetion ofwind direetion, plotted in Fig. 4.52 and Fig. 4.54 is sumrnarized in Ta­ble 4.19.

" 25I..C111 20uL111a.I

x..1:.01~

C

O<Speed< 5m/sO<Speed<10m/s

All dal.a- 10 LJ....L.J-L..l.J...J...J...J..J..J....L...U....I-I-Ju....LJL..U..U-L..LJ.J,.J...J...J..J...J...J..J...J...J...J.J.J....L...U....I-I-J..J..J.J..J

-2:5 -~'" -1:5 -113 -:5 13 :5 10 1:5 20 2:

nlsh\.~ ("pCi'r""'cQlnt." )

Figure 4.52: Windrose at the summit of La Silla in percent of time per sector of 22.5degrees.

Seasonal variations: the average annual eonditions on La Silla are given onFig. 4.53 for day and night as monthly statisties of wind veloeity.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 85

Parameter PARANAL LA MONTURA ARMAZONImin ave max min ave max min ave max

Pressure, mB 734 743 748 707Wind veloeity, m/s 0 7.7 37 0 5.8 a) 23 0 7.3 b) 37

Main noet. wind dir. NNW±25° NE±15° a) NNW±20° b)Humidity I % 1 14 100 1 14 100

Air Temp. (2m), C -8 12.6 22 -8 12.6 22Ground T (-lOem) 3.6 16.5 29.2 1.0 14.4 b) 28.0Ground/air T diff. -18.0 3.8 15.1

Diur. air T var. 1.1 4.0 8.5Noct. air T var.(6h) 0.2 1.3 4.6

Table 4.15: Annual meteorological characteristics in thc Paranal area (sources [2], butfor (a) Avr-Jul1989 (night only on seeing monitor tower); (b) Jan-Fcb 86, Vaisala.

Parameter LA SILLA VIZCACIIASmin ave max min ave max

Pressure, mD 763 770 776 763 770 776Wind velocity, m/s 0 6 41 0 6 41

Main noct. wind dir. NNE±15° N±15°lIumidity , % 2 30 100 2 30 100

Air Temp. (2m), C -8 12.3 22 -8 12.3 22Ground T (-lOem) 3.0 17.4 35.9 3.0 17.4 35.9Ground/air T diff. -18.7 5.1 21.5 -18.7 5.1 21.5

Diur. air T var. 0.9 3.1 7.5 0.9 3.1 7.5Noct. air T var.(6h) 0.2 1.1 4.3 0.2 1.1 4.3

Table 4.16: Annual meteorological characteristics in the La Silla area (sources [2J atLa Silla and Nov88-Jun89 at Vizcachas).

Parameter LE GRAND DENAREmin ave max

Pressure, mB 697 719 724Wind velocity, m/s 0 4.9 24

Main noet. wind dir. East ±15°lIumidity , % 1 53 100

Air Temp. (2m), C -3 8.5 18Ground T (-lOem) 3.6 12.6 21.3Ground/air T diff. -11 3.7 15

Table 4.17: Annual meteorological characteristics on La Reunion (Aug. 86 to May 87).

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Speed up Diurnal, cumul (%) Nocturnal, cumul (%)to (m/s) La Silla Vizcachas La Silla Vizcachas

5.0 52.8 61.0 48.2 67.510.0 82.4 88.5 79.9 89.215.0 96.1 97.4 95.6 98.120.0 99.7 99.5 99.9 100.025.0 100.0 99.9 100.0 100.030.0 100.0 100.0 100.0 100.0

86

Table 4.18: Wind speed at La Silla (sourees Vaisala data, year 1986) and Vizeaehas(Nov 88, June 89).

Angle (center of sector) All data speed $ 10 m/s speed $ 5 m/sSil. Viz. Sil. Viz. Silo Viz.

0.0 (North) 11.6 32.8 12.2 26.7 11.4 18.622.5 33.6 11.2 31.9 11.4 31.3 11.845.0 20.2 6.7 19.7 7.5 19.6 9.367.5 3.8 3.8 4.1 4.4 4.5 5.5

90.0 (East) 1.5 4.0 1.6 2.5 1.9 5.8112.5 0.8 2.3 0.8 2.6 0.9 3.4135.0 1.7 2.7 1.8 3.1 2.0 4.0157.5 5.1 6.9 5.5 7.8 6.0 8.5

180.0 (South) 13.6 8.8 13.8 9.9 12.9 9.4202.5 1.8 1.0 1.9 1.2 2.0 1.5225.0 0.8 0.9 0.9 1.0 1.0 1.3247.5 0.6 0.6 0.7 0.7 0.8 1.0

270.0 (West) 0.7 1.1 0.8 1.2 0.9 1.6292.5 0.7 1.2 0.8 1.3 0.9 1.8315.0 1.0 3.8 1.1 4.3 1.3 4.7337.5 2.3 12.2 2.5 12.7 2.6 11.5

Table 4.19: Wind direetion at La Silla and Vizeaehas in pereent of time per seetor of22.5 degrees.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 87

lSl

m...lSl

.nM

lSl

mM

lSl

.n/IJ

~

~ lSlfv m.... /IJc:>0:CI: lSl

~ .n0:

lSl

m

lSl

vi

lSl

\Si

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

La Silla

MONTH

Figure 4.53: Monthly statistics of wind velocity on La Silla: absolute maximum(*),average of daily maxima(6), average(x), average of daily minima(D).

:; 10 1:5 20 2:

",..CilIULilIa.,x..r.III

c

- Hl U-l...L..L..U-l.J....U....L...L...l..U..L..L..U-l..J....LJ....L...L...l..U...LJ..Ju..J...l..U...LJ..L.l...L.LJ..J..LJ....L..L.W

-2:5 -~12l -1:5 -10 -:5 0

n.L~ht.~ ( .... pQorcCi'nt." )

Figure 4.54: Windrose at the summit of Vizcachas in percent of time per sector of 22.5degrees.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 88

.' Paranal areaAn automatie Vaisala station is operated on Paranal sinee Oetober 84, andwind velocity was monitored at nighttime at La Montura at the top of theseeing monitor tower from April to Oetober 1989.

Table 4.20 gives the wind veloeity distributions measured during the year1986 at Paranal and during the period April 1989-0etober 1989 at La Mon­tura. The eorresponding Paranal noeturnal wind veloeity distribution infunetion of wind direetion is plotted in Fig. 4.56. The wind direetion distri-

Speed up Diurnal, eumul (%) Noeturnal, eumul (%)to (m/s) Paranal Paranal Montura

5.0 31.9 40.3 54.510.0 74.7 75.3 93.815.0 93.2 93.2 99.820.0 98.2 98.2 100.025.0 99.8 100.0 100.030.0 100.0 100.0 100.0

Table 4.20: Wind speed at Paranal (year 1986) and La Montura (Apr. 89 to Oct. 89).

bution at La Montura is given on Fig. 4.58. The differenee with the Paranalwindrose is due to the different shape of the summits themselves.

Seasonal variations: the average annual eonditions on Cerro Paranal aregiven on Fig. 4.57 for day and night as monthly statistics of wind velocity.

An automatie Vaisala station was operated on Cerro Armazoni in Februaryand Mareh 1986, The wind speed at Armazoni may be eompared with datataken simultaneously at Paranal with an identieal meteorologieal station.As shown on Fig. 4.60, the wind eharaeteristies on the two sites agreeremarkably inspite of the 20km distanee.

As a eonsequenee of a larger distanee from the seashore, the rotation of thewind direetion due to thermal eurrents is less important on Armazoni thanon Paranal

Moreover, the similarity between Fig. 4.59 and Fig. 4.56 indieates that theperiod used for eomparison purposes (Feb.-Mareh 1986) was representativeof the yearly pattern in the area.

• La Reunion islandThe windrose at the summit of Le Grand Benare is shown on Fig. 4.55, for10 months between August 86 and May 87, eovering both the dry and wetperiods.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 89

25North

15

20

10

,...5w

I:-I- 0I-It.:J- -5~

I- -10It.:J

z -15x

-20

-25-25 -20 -15 -10 -5 0 5 10 15 20 25

YNIGHT (%NIGHTTIME)

Figure 4.55: Windrose at the summit of Le Grand Benare (sources Vaisala data Aug.86-May 87) in percent of time per sector of 22.5 degrees.

Angle (center of sector) All data speed< 10 m/s speed< 5 m/s0.0 (North) 22.6 18.3 11.7

22.5 9.8 8.3 5.845.0 4.7 5.0 6.577.5 3.2 3.7 6.9

90.0 (East) 3.7 4.4 6.6112.5 4.8 5.5 6.0135.0 6.6 7.0 7.8157.5 2.5 3.0 4.4

180.0 (South) 0.7 0.8 1.5202.5 0.4 0.4 0.8225.0 0.8 0.9 1.7247.5 0.9 1.1 2.0

270.0 (West) 1.8 2.1 3.9292.5 3.2 3.7 5.4315.0 12.8 13.2 11.9337.5 21.5 22.6 17.1

Table 4.21: Wind direction at Paranal (sources Vaisala data Year 1986) in percent oftime per sector of 22.5 degrees.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 90

40

35 NOrh

30

" 25I..C111 20uL111a. 15\

x10..

I:.cnC 5

0

-5

-10-2':5 -al:) -1:5 -l0 -:5 0 :5 10 1:5 20 2:5

nJ.sh"::I , Np~r"'C:Qnt.N )

Figure 4.56: Windrose at the summit of Paranal in percent of time per sector of 22.5degrees.

...~v

IS)

viM

IS)

~M

IS)

viC\J

~

nIS)e

~ ~JJ C\J~I:l: IS)JJ

vi~I:

IS)

~

IS)

vi

IS)

~

Jan Feh Mar Apr May Jun Jul Aug Sep Oct Nov Dec

MONTH

Figure 4.57: Mon~hly statistics of wind velocity on Paranal: absolute maximum(*),average of daily maxima(.6.), average( x), average of daily minima(O).

91

15

20

10

,....5w

1:-I- 0l-XL:J- -S~

I- -10G- -15zX

-20

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

2S

-25-25 -20 -15 -10 -5 0 5 10 15 20 25

YNIGHT (%NIGHTTIME)Figure 4.58: Windrose at the summit of La Montura in percent of time per sector of22.5 degrees.

"\..CGIIJl.GIa. 15\

x10..

t:.Ol-C 5

-5

- 1 0 u...L.......................L.J..J..I.J-I..J....L.JL...U...L.U...I-I-Ju..L.U-L..I.J-I..I-I-JLJ.J..J..W.J....U...J..J...l.l..LLl

-2':5 -2121 -1:5 -10 -:5 0 :5 1121 1::5 2121 2'

nJ.~ht.~ ( ....PQrclilnt. ..... )

Figure 4.59: Windrose at the summit of Armazoni (sources Vaisala data Feb-Mar 86)in percent of time per sector of 22.5 degrees.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 92

gr-l---.--,--,--,-,--,r-r-.-.....,--..,..".....,--,-,--..-r--r.....,-""7\o.siI\J

213.01313

<SIo<SI

ci~--L--l---L......J.-..l-L--J,--'--'--'-..J.....-.L-J--L--'--'--'--.L-J--L-'

13.13130

+

~+ +

<SI + +'" +l'I '"... vi

t:Il: +q;

-w::> 0

'= '"<SIa: .siClww0-

'"<SI<SI<SI

vi

113.131313 .•' 15. 131313SPEED (AVE) PARI\NflL.

Figure 4.60: Simultaneous windspeed measurements at the summits of Armazoni andParanal (sources Vaisala data Feb:Mar.86).

4.4.3 Relative Humidity

Seasonal variations: Fig. 4.61 and Fig. 4.62 present general conditions in thetwo areas of interest during day and night in 1986. For each month, absolutemaximum and average of Ihour averages are plotted.

4.4.4 Temperature

Seasonal variations: Fig. 4.63 and Fig. 4.64 present general conditions in thetwo areas of interest during day and night: for each month, absolute minimum,average of daily absolute minima, average of Ihour averages, average of dailyabsolute maxima and absolute maximum are plotted.

References:[IJ M. Sarazinj Comparison 0/ meteorological conditions on Chilean sites.Annual summary 1985; June 86, VLT Report n048.[2] M. Sarazin; Comparison 0/ meteorological conditions on Chilean sites.Annual summary 1986; January 88, VLT Report n056.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4 3 2 3, ~..: , ,..~ ~..,. ,

! j, ., ', '

f L......... '

La Silla

93

":-t ci~

W lI'l~a:~w:>a:

uitV

citMONTH

Figure 4.61: Monthly statistics of relative humidity on La Silla: absolute maximum(dotted line) and average (fullline). The number of nights with possible condensationis indicated.

cies>

....~ ciW lI'l~a:~w:>a:

uitV

........;- ...... ~ !. .......

: ., ', ', ', '

•••• .! ;

......

Paranal

..... J :.....

MONTH

Figure 4.62: Monthly statistics of relative humidity on Paranal: absolute maximum(dotted line) and average (fullline). The number of nights with possible condensationis indicated.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

~

clM La Silla

94

~

clN

" ~.... cl~

'"~ceCl:

'"~~

cl

~

cl ·Jan Feh Mar Apr May' Jun Jul Aug Se? Oet Nov DeeI

t10NTH

Figure 4.63: Monthly statistics of air temperature on La Silla: absolute minimum(0),average of daily minima(O), average(x), average of daily maxima(ß), absoluteminimum(*).

<SI

clM

" ~~~ cl

'"~ceCl:

'":>ce

<SI

cl

Paranal

<SI

cl1'--...J;;.;;=.J:-':'::".r=:.::..J.:.:=.1=:a..;,.::.:.:J..::..:::.J=u=.:::J:.J.:~~~~--.J

t10NTH

Figure 4.64: Monthly statistics of air temperaturc on Paranal: absolute minimum(0),average of daily minima (0), average(x), average of daily maxima(ß), absolute

\

minimum(*). !

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.5 Seeing

95

4.5.1 Introduetion

Seeing is expressed in arcsec as the Full Width Half Maximum of a stellar imageat the focus of a telescope limited by the atmosphere at O.5/-lm and pointingtowards the zenith.

The output of the Seeing Monitor consists of statistical estimates out of 200short exposures. The corresponding measurement duration varies according tothe number of data invalidated by the various control tests: it is typically 60seconds: Measurements are taken from the differential motion in the direetionsparallel and perpendicular to the pupils separation, which gives two statisticalestimates of the instantaneous seeing. After calibration with the 2.2m telescopein June 88 [2], in the absence of fast (> 20m/s) moving turbulence, it has beenverified that the average of parallel and perpendicular motions lead to an accu­rate measurement of the seeing at the focus of a large telescope. This averageestimate corresponds to the most probable seeing and is the one used in thesite evaluation statistics.

The error budget of the DIMM measurements detailed in section 5.4.1 ex­cludes asymmetrical effects linked to exposure time attenuation or temporaryinstrumental defects. To take into account uncertainties in the symmetry of theinstrumental noise with respect to each direction of measurement, statistics ofthe minimum of the parallel and perpendicular motions are also included in Fig­ures 4.66 and 4.67 as lower limits of the error bars.

Several software updates were implemented in 1988 and 1989 to improve theautomatisation and the measurement accuracy with a continuous follow-up ofthe optical quality of the systems. All DIMMs in operation were updated at thesame time so as to guarantee a constant level of comparability.

As explained in more detail in section 5.4.1, starting in September 1989, in­strumental noise linked to the poor tracking performances of the DIMM portabletelescopes was detected and eliminated on all three systems.

The data obtained during the first period (Oct88-Aug89) are thus equally af­fected in terms of rms differential motion by a non-negligible amount which canbe estimated, as shown by the error bars on Figures 4.66 and 4.67, but not withan accuracy comparable to that of the data of the second period. Yet, becauseof the non-linear dependence of the FWHM with differential motion, the betterseeing is more heavily biased, and differences from site to site are washed out.

Considering the amplitude of the bias, it was decided during the September90 meeting of the Site Seleetion W.G. to further use only the uncontaminateddata for quantitative comparisons of the two sites. Consequently, apart for thequalitative analysis of seasonal variations (§ 5.4.8), the final statistics refer tomeasurements taken after September 89 only.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 96

4.5.2 La Silla area

Seeing at the large telescopes is regularly monitored at La Silla since 1987. Visit­ing astronomers are kindly requested to record image size a few times per nightwhen using CCD detectors. Such seeing statistics are thus repesentative of thecurrent quality of astronomical observations at the observatory, and to controlthe efficiency of various actions taken to reduce dome seeing or to improve theoptical quality of the telescopes.

Due to the fact that they include man made contribution, the seeing averagesgiven in Figure 4.65 are not representative of the intrinsic site quality. Recentcomparisons with the NTT showed that the atmospheric seeing at La Silla wasin fact comparable to what is measured at Vizcachas with the differential imagemotion seeing monitor (DIMM2).

A summary of monthly seeing statistics at Vizcachas is given in Table 4.22for the average of perpendicular and parallel relative motion. The same data areplotted on Figure 4.66 as well as the seeing statistics using the minimum of bothmotions.

4.5.3 Paranal area

Seeing measurements started at Cerro Paranal in April 1987 with the prototypedifferential image motion seeing monitor (DIMM1). Several improvements wererealized in the course of 1987, both to the tower and to the telescope so thata reliable regular operation could be achieved even in that remote part of theworld. Maintenance was placed under the responsability of the La Silla T.R.S.department.

In April 1989, DIMM3 was completed and moved to Cerro La Montura afterhaving been successfully compared with DIMM2 at Vizcachas and DIMM1 atParanal.

Summaries of seeing monthly statistics are given in Tables 4.23 and 4.24 forthe average of perpendicular and parallel relative motions. The same data areplotted on Figure 4.67 for Paranal and Figure 4.68 for La Montura as well as theseeing statistics using the minimum of both motions.

In February 1990, at the request of the site selection Working Group, DIMM3was moved to Cerro Armazoni, after having been used at Las Campanas inDecember 89 and at La Silla in January 90. A comparison of those sites is givenin Section 5.4.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 97

Month Min 5% Max 95% Ave 50% Sigma nr.data nr. nights1988

Oet. Ave. 0.46 0.67 3.21 1.47 1.01 0.95 0.27 8201 28Nov. Ave. 0.35 0.58 2.92 1.64 1.03 0.96 0.34 8131 30Dec. Ave. 0.38 0.63 2.97 1.57 1.04 0.99 0.31 7351 30

1989Jan. ave. 0.43 0.59 2.84 1.77 1.07 0.98 0.38 5447 25Feh. ave. 0.42 0.56 3.19 1.48 0.94 0.87 0.31 7221 27Mar. ave. 0.33 0.52 2.81 1.49 0.93 0.88 0.31 10431 31Apr. ave. 0.34 0.51 3.26 1.42 0.87 0.79 0.31 7199 26May. ave. 0.38 0.48 3.63 1.52 0.86 0.76 0.36 3607 21Jun. ave. 0.29 0.50 2.76 1.35 0.89 0.84 0.29 2003 11Jul. ave. 0.32 0.51 2.72 1.51 0.97 0.93 0.33 3418 20Aug. ave. 0.37 0.53 3.03 1.47 0.92 0.83 0.32 3173 20Sep. ave. 0.32 0.47 2.64 1.39 0.90 0.84 0.29 3105 18Oct. ave. 0.24 0.41 3.67 1.24 0.80 0.75 0.31 5731 26Nov. ave. 0.28 0.47 4.30 1.22 0.78 0.71 0.29 5166 26Dec. ave. 0.27 0.36 2.21 0.91 0.59 0.54 0.20 3694 16

1990Jan. ave. 0.32 0.50 2.13 1.08 0.76 0.72 0.19 4669 25Feh. ave. 0.48 0.57 2.54 1.47 0.90 0.82 0.29 982 8Mar. ave. 0.33 0.45 2.01 1.05 0.70 0.66 0.20 4232 18Apr. ave. 0.31 0.44 2.24 1.02 0.68 0.64 0.19 3287 12May. ave. 0.33 0.42 1.78 1.15 0.77 0.75 0.23 1980 10Jun. ave. 0.34 0.48 2.58 1.30 0.84 0.78 0.28 6908 25Jul. ave. 0.32 0.44 3.08 1.34 0.81 0.74 0.29 3906 15Aug. ave. 0.34 0.47 3.16 1.42 0.87 0.80 0.31 5305 22Sep. ave. 0.37 0.52 2.57 1.55 0.91 0.81 0.33 4077 20

Table 4.22: Statistics of Imn average seeing for the period Oetober 1, 1988 toSeptember 30, 1990 at Vizcachas.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

",., • 1.5 Dcnish telescope)!l ;

i'; o 2.2m telescope

........ 2,

A 3.6m telescope~

, ,:r:: , ;

~ \lJ... ;

'-",

.•4,

0'.8c

'Ci) 1.5 .i~ .;Ul ,t-C .';'0 ..

" " ./~ ..d ,.'~ '. j.

l>"

12 241987 1988 1989

98

Figure 4.65: Monthly median seeing inside La Silla telescopes since Jan. 87 ( CCDmeasurements only, source Astr. Dep. La Silla)

2~~--r-.--r--.---r-....-~.---,--r---r--,-.....,--r-....-..-,......,--r--,--,--,---r--r-~..-,...-,

1.5

•uwU1 "" ... 1

Ua:: '"<I:•~

0U1;l.L

.5

I)

1989 1990

018 24 36 42 48

MONTHFigure 4.66: Monthly statistics of O.51J.m FWHM at zenith measured 5m above groundon Vizcachas, fulllines from top represent 95, 50 and 5 percentiles, stars are for absoluteminima. As seen on thf error bars, starting in September 89, an instrumental noise wasdetected and eliminated, the dotted lines stand for data corrected with the method ofsection 5.4.1.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 99

Month Min 5% Max 95% Ave 50% Sigma nr.data nr. nights1988

Oct. Ave. 0.26 0.55 3.15 1.52 1.01 0.96 0.32 13269 31Nov. Ave. 0.29 0.46 3.98 1.57 0.96 0.90 0.36 11691 30Dec. Ave. 0.30 0.48 3.06 1.37 0.87 0.82 0.29 7959 31

1989Jan. ave. 0.36 0.52 2.69 1.37 0.89 0.84 0.27 9554 29Feh. ave. 0.33 0.52 3.61 1.65 0.95 0.87 0.36 8419 24Mar. ave. 0.34 0.55 2.69 1.29 0.88 0.83 0.24 12527 29Apr. ave. 0.35 0.52 3.79 1.31 0.87 0.82 0.28 9470 27May. ave. 0.26 0.47 2.49 1.33 0.86 0.82 0.27 11763 27Jun. ave. 0.40 0.56 2.34 1.37 0.91 0.86 0.26 12249 27Jul. ave. 0.20 0.56 2.95 1.50 0.97 0.92 0.31 9568 26Aug. ave. 0.29 0.44 2.72 1.51 0.87 0.78 0.35 7379 27Sep. ave. 0.24 0.39 1.88 1.05 0.71 0.68 0.21 5811 23Oet. ave. 0.22 0.35 2.51 1.14 0.69 0.64 0.26 7447 30Nov. ave. 0.27 0.38 2.18 1.l1 0.68 0.64 0.24 7845 30Dec. ave. 0.22 0.38 2.23 0.99 0.64 0.58 0.21 9180 30

1990Jan. ave. 0.30 0.40 2.90 1.10 0.69 0.64 0.25 8227 30Feh. ave. 0.22 0.36 2.47 1.14 0.69 0.64 0.26 6909 25Mar. ave. 0.18 0.43 2.70 1.16 0.71 0.65 0.26 8254 30Apr. ave. 0.31 0.45 2.12 1.17 0.73 0.67 0.24 5741 24May. ave. 0.26 0.37 2.35 1.06 0.65 0.59 0.23 6632 27Jun. ave. 0.21 0.34 2.43 1.05 0.64 0.59 0.23 4450 21Jul. ave. 0.21 0.36 2.70 1.18 0.66 0.58 0.29 3913 24Aug. ave. 0.19 0.31 2.36 0.92 0.57 0.52 0.21 6494 27Sep. ave. 0.19 0.27 2.35 1.23 0.65 0.58 0.31 7510 29

Table 4.23: Statisties of lmn average seeing for the period Oetober 1, 1988 toSeptember 30, 1990 at Paranal.

Month Min 5% Max 95% Ave 50% Sigma nr.data nr. nightsApr. ave. 0.38 0.56 4.05 1.66 1.03 0.93 0.40 6236 25May. ave. 0.27 0.43 3.00 1.62 0.88 0.77 0.40 8160 27Jun. ave. 0.36 0.52 3.04 1.75 1.01 0.89 0.40 7529 26Jul. ave. 0.35 0.50 3.56 2.01 1.11 0.97 0.49 4560 20Aug. ave. 0.30 0.45 3.26 1.62 0.89 0.77 0.40 7222 29Sep. ave. 0.27 0.48 3.20 1.83 1.00 0.86 0.44 8615 18Oct. ave. 0.28 0.41 3.51 1.41 0.77 0.66 0.34 9491 31

Table 4.24: Statisties of lmn average seeing for the period April 1, 1989 to Oetober31, 1989 at La Montura.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 100

1.5

. 5

I....·

1.... ,..1 .,.......1'" ..

MONTH

Figure 4.67: Monthly statistics of 0.5J.1.m FWHM at zenith measured 5m above groundon Paranal, fuUlines from top represent 95, 50 and 5 percentiles, stars give absoluteminima. As seen on the error bars, starting in September 89, an instrumentalnoise wasdetected and eliminatedj the dotted lines stand for data corrected with the method ofsection 5.4.1.

2

1.5L...J

,uw~ 10:<r,

~1:5 .5Li..

8

nrrn~I)L ...,I) 1),.....:

I) 1)1)1)

1989 1990o

18 24 30 MONTH 36 42 48

Figure 4.68: Same as Fig. 4.67 for measurements at La Montura from April to October1989.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.6 Scintillation and upper atmospheric wind

101

4.6.1 Introduction

The altitude distribution of the turbulent layers plays a role in the size of theisoplanatic angle. For a given seeing, the angular field inside which the wavefrontdistortions remain correlated is smaller when the turbulence occurs at high alti­tude. This parameter is also qualitatively related to the index of scintillation. Inother words, a site showing less scintillation is bound to offer a larger isoplanaticangle.

In the same manner, the velocity distribution of the turbulence determines thespeckle lifetime, or the time during which the wavefront spatial characteristicsremain correlated. For a given seeing (even very good), fast moving turbulentlayers reduce the coherence time and prohibit the use of modern astronomicaltechniques such as high resolution imaging by interferometry.

4.6.2 Radiosonde data

Inspite of the fact that there is no direct link, at ground level, betwecn windspeed and thermal turbulence, it is believed that high wind shear created bythe jet stream (where the speed may exceed 60m/s) increases its probability. Astatistical analysis of the mean wind speed in the 11 to 12km altitude range givesthe required information.

Upper air observation digital files have been purchased from the National Cli­matic Data Center (NCDC, Asheville, North Carolina, USA). These files are re­lated to meteorological balloons launched twice a day from Lima (Peru), Antofa­gasta, and Quintero (Santiago) stations during the years 1958 to 1984.

Table 4.25 presents the monthly average of wind velocity at the 200mB altitudelevel in m/s for the three stations. One should note the high value of standaradeviations related to large day to day variations due to the North-South motionof the core of the jet stream (West-East winds at the tropopause level).

Paranallies elose to Antofagasta (120km South) but La Silla is located betweenQuintero and Antofagasta (1/3, 2/3). As shown in Fig. 4.69, we want to pointout aglobai seasonal effect linked to eloseness to the equator. Though it is acrude approximation, a value of the wind over La Silla may be obtained by alinear interpolation between the values at the two stations.

4.6.3 Scintillation

The index of low frequency « 50Hz) scintillation is monitored by the DIMMout of each series of 200 CCD exposures. The value obtained is much lowerthan the fuH range (1000Hz) but its seasonal variations may be an indication ofthe frequency of occurrence of turbulence in the high atmosphere. In referenceto current attempts to link wind velocity to refractive index structure ([1] [2]),

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 102

35 Table: sstat wind I-"-'{r-? cr-----O--'-'iG ... ··.qJ : 6----6 ."".qJ

30 eb-..... .qJ - -- G·.. ·· G ..... -g.....:·..·.-e-..... ---I~ ..... "[;)-..... <!l --

..... -rb25

,Q

eo~ 20"0I::":;

15

10

1055L---I.._....l-_.l...----I._.....l-_...L..----l_-l-_...L----:L---1-_..L-----J

oMONTH

Figure 4.69: Seasonal variation of the wind speed at 200mB over Lima (fuU line),Antofagasta (dots-o) and Quintero (dashes-D) from 1958 to 1984.

Fig. 4.70 shows a comparison of monthly averages of radiosonde measurements at200mB from the Chilean Antofagasta station and of the index of low frequencyscintillation at Paranal for the period 1987-1990.

As for a site to site comparison, cross-calibrations are necessary because ofthe different level of noise of intensified CCDs. We found that, as shown onFig. 4.71, two different systems working side by side give wen correlated val­ues and that the correction in terms of index of scintillation is multiplicative(currently DIMM2=0.88DIMM3=0.68DIMM1).

References[1] C.E. Coulman, J. Vernin, Y. Coqueugniot and J.L. Caccia, Guter scale 0/turbulence appropriate to modeling re/raetive-index strueture profiles; App. Opt.Vo1.27, nOl, January 88.[2] LASSCAj La Silla Seeing Campaign, Data analysis Part II, Speckle li/etime, Isoplanatic angle and Guter scale 0/ turbulence; December 1990, VLTReport n060.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 103

48

o

423630

00

126

. 1

.08

[T).06'"'"0...-<

ui+J0

.04c~

'0C

:).020

0.0l-.+J

00 18 24

MONTH

Figure 4.70: Comparison ofthe monthly average of wind velocity at the tropopause overAntofagasta (full line) with the index of low frequency scintillation at Cerro Para.nal(dashes) since Jau. 1987.

. 2

. 15 SClntlllatlon: OIMM3=0.7730IMM1'+0·•. 000

L.,nlPo;:; . 1c

. 05

o .05

~ r • .. ,, .

.:' .

"

. 1 .15Indlslo

Figure 4.71: Comparison of simultaneous measurements of low frequency (50Hz) scin-tillation on two side by side CCD systems on Paranal, June 90.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

Month Station Sampies wind speed std. dev.Jan. Lima 86 14 6.8

Antofagasta 183 19 9.4Quintero 187 32 13.0

Feh. Lima 71 12 5.4Antofagasta 164 16 8.4

Quintero 165 30 11.1Mar. Lima 87 10 5.0

Antofagasta 181 22 9.9Quintero 191 29 11.1

Apr. Lima 84 13 7.1Antofagasta 192 30 12.0

Quintero 168 26 12.7May. Lima 83 19 8.8

Antofagasta 172 29 13.7Quintero 151 30 12.0

Jun. Lima 77 17 7.6Antofagasta 158 34 14.3

Quintero 117 34 12.0Jul. Lima 83 17 7.9

Antofagasta 173 32 13.2Quintero 143 31 11.5

Aug. Lima 84 16 8.7Antofagasta 170 32 12.9

Quintero 147 30 11.9Sep. Lima 83 13 7.6

Antofagasta 162 34 13.7Quintero 159 31 11.6

Oct. Lima 83 13 6.8Antofagasta 187 34 12.3

Quintero 152 31 11.6Nov. Lima 76 14 7.2

Antofagasta 216 30 10.8Quintero 168 32 12.0

Dec. Lima 72 15 7.4Antofagasta 170 25 9.5

Quintero 169 30 12.5

Tahle 4.25: Wind speed at 200mB from radiosonde data.

104

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

4.7 Ground level turbulence

105

The vertical distribution of ground turbulenee (10m-30m) was measured at LaSilla during LASSCA in February 1986, and the average Cl at 10 met~rs was2.710-3 J(2 .m-2/ 3 (see [1] p 88).Mierothermal sensors were used from June 1987 to Mareh 1989 at Paranal wherethe average value of the temperature strueture parameter at 5m above groundwas 2.910-3 J(2. m -2/3.

The same measurements were made at La Montura from July to September1989 where the ground turbulence was four times higher with an average of12.310-31(2 .m-2/ 3 •

An automatie mierothermal monitoring station was designed by the TRS de­partment at La Silla for Cerro Armazoni where measurements started in July90: the values measured at 2, 5 and 8 meters above ground were respeetively 12,8.5 and 5.1 1O-31(2.m-2/3. All those measurements are summarized in Fig. 4.72,and eompared to a numerieal simulation made at Risoe Laboratories using a 2Dmodel of a neutral flow on an isolated mountain top [2].

A seeond interesting feature is the daily variation of ground turbulenee obtainedat Armazoni and shown on Fig 4.73. The eontribution of the 2m-5m layer to theseeing is found to be 6 times larger during daytime.

References[1] LASSCAj La Silla Seeing CampaignJ Data analysis Part IJ Seeing; .Deeember 1987, VLT Report n055.[2] Anne F. de Baas; Modelling of the temperature structure funetion; Esoeontraet 28781/5219/VLT/02, Item 2.2, Phase 1 report, Risö Laboratories.

4.8 Dust

4.8.1 Laboratory tests

Before sending the equipment described in 3.9 to La Silla, the particle counterwas used intensively at ESO Garehing, to aequire experience as weIl as usefuldata.

A laminar air fiow box dass 100 was used as a referenee for ealibration. Class100 is reached when particle eount does not exceed a total of 100 particles percubie foot (3.5 particles per liter) of a size 0.5p,m and larger (Federal StandardUS 209B). Measurements performed during February 1987 gave the followingresults:

Outside t = -5 C no windOffice roomOptieal laboratory: climatisation OFF

526 224 pes.401 065 pes.295 585 pes.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

.012 X 83 X Armazonl

EE t'ontura

.01 [8J Paranal

0 Lä Sllla

X~ .008l'l,NIE

N~.006N

'"'u

.004

.002

106

o 10 20 30 40 50helght (meter)

Figure 4.72: Temperature structure parameter Ct 2 versus height above ground levelon various sites and compared to Risö numerical model (fullline) .

.'..". .... .

o 5 10 15 20TIME (Hours.U. Tl

Figure 4.73: FWHM contribution of the ground layet between 2m and 5m versus timeof the day (dot = 1 minute average). The fuU line represents the sun radiation inkwjm2 , vertical ~nomalies are due to shading by the mast tubing. Shortly after 12hUT, the transition from nighttime stable stratification to daytime convection inducesa moment 0/ silence during which one detects virtually no turbulence whatsoever.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN 107

Opticallaboratory: climatisation ONInside climatic chamber with new filtersLaminar air flow: optieallab.

66345 pes.6595 pes.40 pes.

4.8.2 Measurements inside the telescopes

Several interesting measurements have been performed at La Silla:-General quantification of the dust pollution and the possible day-night evolu­

tion at locations undisturbed by human aetivity.-Measurements inside and outside the telescope domes: aseries of measure­

ments have been conducted at the 3.6 telescope, namely near the primary mir­ror, the ventilation units and outside, on the eatway. It was possible to verify theeffect of the internal ventilation system. Measurements were also taken in the 2.2telescope in order to verify possible effeets on the dust coneentration due to thestable stratification obtained by the eooled floor:

Outside of NTT - ramp levelOutside of Schmidt teleseopeInside 2,2 m telescope: domeInside NTT, Nasmyth room c1ima OFFInside 3,6 m - dome openOutside 3,6 m

22 937 pes.20 899 pes.35 500 pes.52 946 pes.56 270 pes.56 986 pes.

4.8.3 Paranal area

A eomplete month of night measurements has been recorded in September 1989at La Montura. The measurements were taken at two different heights aboveground, 1m and 7m. However no significant difference appears between the twolevels and both are considered in the nightly averages. These are summarized inFigure 4.74, normalized to the referenee volume of one cubic foot.

One may note that most of the time, the dust quantity is weH below thevalue of 10000 partic1es per cubie foot which represents the standard for a cleanenvironment. No obvious relationship was found between dust quantity and windvelocity or air temperature.

4.8.4 La Silla area

Measurements were taken during 14 nights in August 1989 at Cerro Vizcachasin the conditions deseribed above. The summary of nightly averages is presentedon Fig. 4.75.

CHAPTER 4. RESULTS OF THE TEST CAMPAIGN

DUST SURVEY: LA MONTURA

..

*~ ..JUHl­n:[~

SEPTEMBER lsasin Na.., , •••

I .•';lA!§! I

108

Figure 4.74: Nocturnal average number of dust particles per minute at La Montura.

DUST SURVEY: LAS VISCACHAS

..'t1I1+W"1I1

lIl ..WJUHl-n:[~

AUGUST lsas

\\\. .

'\ ',ß'. . \. . .. .,\ . .

· '. . .· \ ..\ .· ". .21' Nov &5••

Figure 4.75: No~turnal average number of dust particles per minute at Vizcachas at5m above ground.

Chapter 5

ANALYSIS

5.1 Cloudiness

5.1.1 Accuracy of the measurements

Sinee the beginning of September 1983, cloudiness at the summit of Paranalhas been monitored eontinuously, every night and day (Ardeberg et al., 1990a).During night time, inspection of photometrie sky quality has been made everyhour, during daytime every second hour. During night time, corresponding cloudeover monitoring has been made at La Silla. Local differential comparison hasdemonstrated that the cloud cover data obtained for Paranal is valid also forthe higher summits in the neighbourhood of Paranal, including Armazoni. In thesame way, it has been shown that the cloud cover data obtained at La Silla canbe used also for Vizcachas.

In addition it should be noted that several years of cloud cover monitoringexist for La Silla, starting from 1965. Inclusion of these data improve our basisfor an evaluation of the existence and significance of possible long-term trends. Ifexisting, such trends are an important part of our data on site quality parameters.

The size of our data sampies is favourable not only with respect to studies ofpossible long-term trends. In addition, it significantly improves our abilities totest the quality and consistency of cloud cover evaluations, aHowing us to comparedata from sub-samples which are defined differently and still of significant size.

Even weH trained and motivated observers will inevitably experience difficul­ties with faint clouds during dark nights. Thus, it is essential to estimate thesignificance and consistency of this effect. Given a large sampie of data, the mostimmediate way to obtain such an estimate is through comparison of data fromperiods centered on fuH moon and on new moon, respectively. Whilst also veryfaint clouds will be safely recorded by trained observers during nights around fuHmoon, such clouds easily escape detection during nights around new moon.

Also in the case of a large sampie of data, undersampling for differential com­parison of fuH-moon and new-moon data may weH give rise to some vulnerability

109

CHAPTER 5. ANALYSIS 110

to effects of larger weather patterns. For this reason, an increase in reliability isvaluable and can, in the case of Paranal observations, easily be obtained throughthe inclusion of daytime data. As a whole, we assurne that there is no statisti­cal relation between moon phase and cloud cover, and, further , that the diurnalcloud cover cycle is independent of moon phase. Both assumptions seem highlynatural.

For the cloud cover data obtained at Paranal, we then compare the differencebetween nighttime and daytime cloud cover as observed during full-moon andnew-moon periods, respectively. For this comparison, we make use of the obser­vations collected during the seven days and nights surrounding full moon andnew moon, respectively. The comparison is made in terms of the difference inthe percentage of photometric nights and days. As for La Silla no daytime dataare available, we have, in this case, to rely on a comparison of nighttime dataonly, as obtained around full moon and new moon, respectively. In principle, thisshould yield equivalent, although somewhat less accurate, data.

It is recalled that the definition of a photometrie night calls for six hoursof consecutive photometric night time, where photometrie time is used todescribe time without any cloud in the sky higher than five degrees above thehorizon. Nighttime in this sense is restricted to hours for which the Sun has analtitude lower than 18 degrees below the horizon.

Correspondingly, daytime is taken to mean the time for which the Sun hasan altitude of more than 18 degrees above the horizon. Hours outside those ofnighttime and daytime are referred to as morning and evening, respectively.

Similarly, a speetroseopie night/day is defined as one for which the skyhas been spectroscopic during a total of at least 6 hours, with no demand onconsecutiveness. Speetroseopie sky is defined as one with at least 50% of itsarea free from clouds 01' at most 20% absorption over a major part.

For Paranal, results of moon testing of the quality of cloud cover data are givenin Table 5.1. The difference between the percentage of photometric nights andphotometrie days is given for new-moon and for full-moon time, respectively,as weIl as the resulting overall difference between these differences. In the sametable, for La Silla, the percentage of photometrie nights is given for new-moonand full-moon time, respectively, together with the difference between these per­centages.

Table 5.1 demonstrates two facts. First, it seems confirmed that during new­moon periods, a certain amount of faint clouds tends to escape detection. Second,this effect seems to be very much similaI' at Paranal and at La Silla. Thus, forcomparisons of these sites, the effect is of no importance. Further, the effect canbe fully excIuded, if one takes into account only data obtained around full moon.Similarly, with the comparisons available, a statistieal correction can be made.Taking also into i~ccount periods between full and new moon, the correction tobe applied to the eomplete cloud cover material should be around two to three

CHAPTER 5. ANALYSIS

pereent. Below, observed data are given without eorreetions.

111

5.1.2 Cloudiness in the La Silla and Paranal areas

For the evaluation of photometrie sky quality, we have, throughout the site eval­uation eampaign for the ESO VLT, employed two quality eoneepts, referred toas photometrie quality and speetroseopie quality, respeetively. Coneerning thedefinitions of these eoneepts, details have been given above. Expericnee from in­tereomparisons of evaluations made by different experienecd observers show theeoneept of photometrie sky quality to give very eonsistent results. Thus, we rc­gard our data on photometrie sky quality as solid. That this holds also for darknights has been shown above. Intereomparison of evaluations made by differentexperieneed observers eoneerning the quality of the sky when partially eoveredby clouds shows the eoneept of speetroseopie sky quality to give less eonsistentresults than the eorresponding eoneept of photometrie sky, with personal equa­tions as weIl as statistieal spread of the data being more pronouneed in ease ofspeetroseopie sky quality than in the ease of photometrie sky quality. This impliesthat the sky quality domain enclosed between, on the one hand, fully clear toslightly cloudy, and, on the other hand, nearly fully cloudy to fully cloudy, eannotbe evaluated in a manner whieh gives a eonsisteney eomparable to that distin­guishing between photometrie and non-photometrie sky quality. This is taken asa strong indication that higher weight should, in general, for evaluations of sitequality, be attaehed to photometrie sky quality than to speetroseopie sky q~ality.

Regarding photometrie sky quality, monthly pereentages of photometrie nightsare shown in Figure 5.1 for La Silla, dashed eurve, as well as for Paranal, full­drawn eurve. The period eovered is from September 1983 to November 1989. Theeorresponding differenee in monthly pereentages of photometrie nights betweenParanal and La Silla is plotted versus time in Figure 5.2.

For the same time period as eovered by Figures 5.1 and 5.2, Figure 5.3 shows forLa Silla, dashed eurve, and for Paranal, full-drawn eurve, monthly pereentages ofphotometrie nights passed through a time filter with a width of 12 months. Theeorresponding time filtered differenee in monthly pereentages between Paranaland La Silla is givcn in Figure 5.4.

Valid for the period from September 1983 to November 1989, average seasonaleycles of pereentage of photometrie nights are displayed in Figure 5.5 for La Silla,dashed eurve, and Paranal, full-drawn eurve.

With respeet to spcetroseopie sky quality, Figure 5.6 shows monthly pereent­ages of speetroseopie nights for La Silla, dashed eurve, and for Paranal, full-drawneurve. As in Figure 5.1, the period eovered is from September 1983 to Novem­ber 1989. In Figure 5.7, the eqrrespondin~differenee in monthly pereentages ofspeetroseopie nights between Paranal and' La Silla is plotted versus time.

Figure 5.8 displays, for the same time period as included in Figures 5.6 and 5.7,monthly pereentages of speetroseopie nights passed through a time filter with a

CHAPTER 5. ANALYSIS 112

100

80

- ,aoc ,.....

I " 160 • "ZI ~ " ,0 , ' ,H , .,\ "

I- t.: ,,'tJ ,':< 40 ". .a: ..:'u.

" "" .., .I

20

01983 1984 1985 1986 1987 1988 1989

YEAR

Figure 5.1: Monthly pereentages of photometrie nights for the period 1983-1990. Datafor La Silla are indieated by a dashed eurve, those for Paranal by a full-drawn eurve.

70

50

UJ 30tJZUJa:UJlt 10Ho

-10

-30 l..-........L_-I-_..L----I_......._....I-_l..-........L_-'-_..L----I_.......---'

1983 1984 1985 1986 1987YEAR

1988 1989

Figure 5.2: Diffetenee in monthly pereentages of photometrie nights between Paranaland La Silla for the period 1983-1990.

CHAPTER 5. ANALYSIS 113

100

zoI-tI­U<a:LI.

80

60

40

20

1984 1985 1986YEAR

1987 1988 1989

Figure 5.3: Monthly pereentages of photometrie nights from 1983 to 1990, passedthrough a time filter having a width of 12 months. Data for La Silla are indieatedby a dashed eurve, those for Paranal by a full-drawn eurve.

70

50

EUJ 30uzUJa:UJLI. 10LI.I-t0

-10

-301984 1985 1986 1987 1988

YEAR

Figure 5.4: Differenee in monthly pereentages of photometrie nights betweell Paranaland La Silla, passed through a time filter having a width of 12 months. The periodeovered is from 1983 to 1990.

CHAPTER 5. ANALYSIS 114

100

80

zoI-t.­Uc(a:u..

60

40

20

, , " .." ,,'" '," \ ,""r \ "I , I

I , ", \ ,....... I

I '" ..

'.. "........ ---- .... '

o N 0 J F M A M J JAS 0 N 0 J F MMONTH

Figure 5.5: Average seasonal eycles in pereentage of photometrie nights for the period1983 to 1990. Data for La Silla are indieated by a dashed eurve, those for Paranal bya full-drawn eurve. The diagram covers a time interval of 18 months.

width of 12 months. Results for La Silla are indieated by a dashed eurve, thosefor Paranal by a fuIl-drawn eurve. In Figure 5.9 , we show the eorresponding timefiltered differenee in monthly pereentages of speetroseopie nights at Paranal andat La Silla.

Average seasonal eydes of the pereentage of speetroseopie nights are shown inFigure 5.10 for the period from September 1983 to November 1989. Results forLa Silla are indieated by a dashed eurve, whilst those for Paranal are indieatedby a fuIl-drawn eurve.

In order to evaluate the possible presenee of long-term trends of the sky qualityat La Silla and at Paranal, suitable data are provided by Figures 5.1 to 5.4, in theease of photometrie nights, whilst eorresponding data for spectroseopic nights arefound in Figures 5.6 to 5.9. These data indicate a eertain variation in photometriesky quality with an amplitude of the order of twenty pereent for La Silla andfifteen pereent for Paranal. The eorresponding variations in speetroseopic skyquality show an amplitude of the order of 15% for La Silla and 10% for Paranal.Both sites seem to have a common typieal variation period of around three years,in ease of photometrie as weIl as in ease of speetroscopie sky quality. In the easeof photometrie sky quality, La Silla and Paranal seem to have a eommon phase,whilst this seems( less dear in the ease of speetroseopic sky quality. As a whoie,a smaIl phase sh~ft seems indieated between photometrie and speetroseopie skyqualities. Interestingly, also small-scale patterns in the photometrie sky quality

CHAPTER 5. ANALYSIS 115

are weIl eorrelated for the two sites.At the same time as there seems to be solid evidenee in favour of a eertain

variation in photometrie and speetroseopie sky quality at La Silla and Paranalwith aperiod around three years, there is no signifieant evidenee for a long-termtrend. The absence of evidenee in favour of long-term trends is rather deal' fromFigures 5.1, 5.4, 5.6 and 5.9 . In addition, Figures 5.2, 5.4, 5.7 and 5.9 seemto indieate that any long-term trend in photometrie and/or speetroseopie skyquality, if at all present, will be eonsiderably redueed as soon as differential datafrom Paranal and La Silla are regarded.

The absence of signifieant long-term trends in photometrie and speetroseopiesky quality at (and between) La Silla and Paranal emphasizes the value of anexamination and eomparison of the eorresponding seasonal eyeles at the twomountains. Relevant data are provided in Figures 5.5 and 5.10, respectively.

Turning first to photometrie sky quality, the data shown in Figure 5.5 givea number of solid indieations. First of all, it is beyond doubt that Paranal hasan overall photometrie sky quality eonsiderably superior to that of La Silla.With a peak differenee between the two eurves elose to 40 percent, La Silla doesnever outperform Paranal. Seeondly, photometrie sky quality has very differentseasonal behaviours at La Silla and Paranal. La Silla has a photometrie qualitywith a pronouneed seasonal eyele, whilst seasonal variations at Paranal are ratherlimited. This is, in its own right, a strong argument in favour of the Paranal areaas eompared to the La Silla area. Finally, a eertain eorrelation between the twosites seems to be present regarding month-to-month variations.

Coneerning speetroseopie sky quality, some indieations are given by the datadisplayed in Figure 5.10. As for the photometrie sky quality, it is evident thatParanal has an overall speetroseopie sky quality superior to that of La Silla. Witha peak differenee of dose to 30% in favour of Paranal in July, La Silla outperformsParanal only in January and February. Again as for photometrie sky quality, theseasonal behaviour of speetroseopie sky quality is very different at La Silla and atParanal. La Silla has a pronouneed seasonal variation in speetroseopie sky quality,albeit less heavy than in the ease of its photometrie sky quality. At Paranal,the speetroseopie sky quality shows quite limited seasonal variations, with apattern resembling that displayed by the eorresponding variations in photometriesky quality. In both eases, an indieation of a double-peaked strueture might beinterpreted as resulting from the relatively small distanee of Paranal from theequatorial elimate region.

It should be emphasized that eareful mOllitoring of sky quality has demon­strated that the eonelusions diseussed regarding a eomparison of photometrieand speetroseopie sky quality at La Silla and Paranal ean be safely interpretedas valid for the La Silla and Paranal areas as weIl.References: see Chapter 2

CHAPTER 5. ANALYSIS 116

100 , ,, I, I, ,80 I ,

I ,I 1.' It ' 'I' ,. ,"

E I ,I, 1,1 1

1',\ ' " 1,' , :60 " ,I ~ " I ",

Z I, ,I t •,.0 , I I,1-1 "~ "u

~<C 40CI:IJ..

20

01983 1984 1985 1986 1987 1988 1989

VEARFigure 5.6: MOllthly percentages of spectroscopic nights for the period 1983-1990. Datafor La Silla are indicatcd by a dashed curve, those for Pal'anal by a full-drawn curve.

70

50

EllJ 30uZllJCI:llJIJ.. 10IJ..1-10

-10

1983 1984 1985 1986 1987VEAR

1988 1989

Figul'e 5.7: Differipllce in monthly percentages of spectroscopic nights betwecll Paranalalld La Silla for tllC period 1983-1990.

CHAPTER 5. ANALYSIS 117

100

40

,----,.. _---.. "80 ~;,. ,-..... '

'~-~,-~ -,,-,,-,,;,;,_;,,~I

60zoH...U<a:u.

20

o ~---l:""----I_---I._--L_--L_--L._--L._--l-_.....L_..l-_.J

1984 1985 1986 1987 1988 1989YEAR

Figure 5.8: Monthly percentages of spectroscopic nights from 1983 to 1990, passedthrough a time filter having a width of 12 months. Data for La Silla are indicated bya dashed curve, those for Paranal by a full-drawn curve.

70

50

ELU 30uZLUa:LUu. 10u.H0

-10

-301984 1985 1986 1987 1988

YEAR

Figure 5.9: Difference in monthly percentages of spectroscopic nights between Paranaland La Silla, passed through a. time filter having a. width of 12 months. The periodcovered is from 1983 to 1990.

CHAPTER 5. ANALYSIS 118

100,,

,,,80 , ,,,.. ',"

E \,

\ ,,60 \,

Z0....I-UCl: 40CI:LL

20

0o N 0 J F M A M J JAS 0 N 0 J F M

MONTH

Figure 5.10: Average seasonal eycles in pereentage ofspeetroseopie nights for the period1983 to 1990. Data for La Silla are indieated by a dashed eurve, those for Paranal bya fuH-drawn curve. The diagram covers a time interval of 18 months.

Paranal La SillaMoon phase % phot.nights-% phot.days % phot. nights

New 19.3 58.1FuH 11.3 54.8

(new)- (fuH) 7.9 3.3norm to La Silla 5.6 3.3

Table 5.1: Results of moon phase testing of the quality of c10ud cover data. For Paranal,the difference is given between the pereentage of photometrie nights and photometriedays as observed at new moon and fuH moon time, respeetively. For La Silla, forwhieh no daytime sky quality observations are available, thc pereentage of photometrienights is given for new moon and fuH moon time, respeetivc1y. In both eases, resultingdifferenees between pereentages at new moon and fuH moon are shown. The last eolumngives these differellees normalized to the level of photometrie nights at La Silla. Theseare the data to be )compared.

CHAPTER 5. ANALYSIS

5.2 Precipitable Water Vapour in the Atmosphere

119

5.2.1 Accuracy of the measurements

Calibration of our measurements of integrated atmospheric water vapour hasbeen made in two steps. First, internal calibration has been made with the helpof liquid nitrogen. Second, absolute calibration has been made through simul­taneous measurements with monitors and high resolution spectroscopy of atmo­spheric water vapour absorption. Both calibration steps have been detailed insection 3.3.3, as weIl as the resulting accuracy.

Some comparisons have been made between our data for integrated atmo­spheric amounts of precipitable water vapour and corresponding data derivedby other methods. These other methods include ascents of radiosondes withhumidity sensors and measurements of water vapour extinetion coefficients atsubmillimetre wavelengths.

Over aperiod of 26 days in the beginning of 1986, the amounts of precipitableintegrated atmospheric water vapour were compared as obtained from our datacolleeted at the summit of Paranal and as derived by the meteorology departmentin Antofagasta, respectively. The meteorology department used daily ascents ofradiosondes with humidity sensors to measure the amount of precipitable watervapour above Antofagasta. In order to obtain compatible data, we used, for ourintegrations, only the radiosonde humidity data colleeted at altitudes higher thanthat of the summit of Paranal. For such altitudes it seemed reasonable to assurne,in a first approximation, that results obtained at Paranal and at Antofagasta,both coastal places with a North-South distance of the order of 120 kilometres,should be comparable.

The comparison between our data on amounts of integrated atInospheric wa­tel' vapour and corresponding data from ascents of radiosondes with humiditysensors at Antofagasta showed good zero-point agreement but relatively high dis­persion. There was no significant difference between the zero-points of the twosystems, a formal difference of +0.19 ± 0.15 millimetre of precipitable water. Inprinciple, then, we measured a marginally higher amount of water vapour thanwas measured by the meteorology department of Antofagasta. For a single mea­surement comparison, the corresponding standard deviation was 0.96 millimetreof precipitable water. We are fully convinced that the major part of the scatteris due to uncertainties from the radiosonde data. Chilean meteorology expertsagree with these conclusions.

At La Silla, a thorough and rather detailed comparison was made betweendata on precipitable atmospheric water as measured with our sky radiance mon­itors and as derived from data obtained at submillimetre wavelengths by theBonn group under A. Schulz. This comparison, based on an extensive material,showed an excellent agreement between the two sets of data on prccipitable at­mospheric water vapour both regarding zero-point and dispersion. We note, that

CHAPTER 5. ANALYSIS 120

the possible small (around 0.2 millimetre) zero-point uncertainty resulting fromcalibration with liquid nitrogen (Schulz, 1990) is fuIly eliminated after our abso­lute calibration through measurements of atmospheric water vapour absorptionat high speetral resolution.

Also at Paranal, detailed comparison was made between data on precipitableatmospheric water vapour as derived from measurements made with our skyradiance monitors and from data obtained at submillimetre wavelengths by R.Martin. A more detailed analysis of this comparison is still pending. Evidently,the result is more complicated than those of the comparisons described above.Further detailed comparisons of the rich data available as weIl as further analysisseem necessary before a final conclusion can be reached in this case.

We conclude, that available results of comparisons indicate that our results arecompatible with those obtained through different techniques. Especially the re­sults from the detailed comparison with data from measurements at submillime­tre wavelengths by the Bonn group strongly indicate high reliability of zero-pointas weIl as of system consistency. This should assure that our data on integratedatmospheric water vapour should be of an accuracy weIl abovc that necessaryfor a differential comparison of the site candidates for the ESO VLT, especiallysince the summits in question aIl have comparable altitudes, with the higher onesbeing situated doser to the equator.

5.2.2 Precipitable Water Vapour above La Silla and Paranal

A comparison of the distributions of the amount of precipitable atmosphericwater vapour above La Silla and Paranal during night time is given in Figures 5.11to 5.13. The data included correspond to the period 1983 to 1989, inclusive.Figure 5.11 shows the situation for winter nights, Figure 5.12 that for nightsduring spring and fall, whilst Figure 5.13 details that of summer nights. Data forLa Silla are indicated with dashed histograms, those for Paranal with full-drawnhistograms. For the definition of winter, spring and fall, and summer, see figurecaptions. In Figures 5.14 to 5.16, we display equivalent comparisons as given inFigures 5.11 to 5.13 but for data obtained during daytime.

In Figure 5.17, a comparison is made of the seasonal variations of averageperiodical amounts of integrated atmospheric water vapour above La Silla andParanal during nighttime. Average amounts of precipitable water vapour havebeen plotted for periods of ten days, accumulated from 1983 to 1989. Data forLa Silla are indicated with a dashed curve, those for Paranal with a full-drawncurve. Figure 5.18 shows data equivalent to those given in Figure 5.17 but validfor daytime.

From the comparisons shown in Figures 5.11 to 5.18 , it is evident that thesummit of Parahai has an atmosphere with an integrated content of water vapourconsiderably smaller than that of La Silla. Further, whilst favourable conditionswith respeet to atmospheric water vapour are frequent at both places during

CHAPTER 5. ANALYSIS

50

40

121

10

IIIII__ J

I---~

I II I

IIIIII

o 2 4 6 6 10 12WAlER VAPOUR CO~~ENT (mm)

Figurc 5.11: A comparison of thc distributions of amounts of water vapour in theatmospheres above La Silla and Paranal during winter nights. Winter time is definedas lasting from June 1 to September 30, both limits inclusive. The distributions, inpercentages, are given for the amount of predpitable water, expressed in millimetres.Data for La Silla are indicated by a dashed histogram, those for Paranal by a full-drawnhistogram.

winter time, this is not the case during the rest of the year, especially in sum­mer time. Whilst Paranal offers periods with low amounts of atmospheric watervapour also during the spring, fall and summer, this is more than doubtful re­garding La Silla.

La Silla and Vizcachas have nearly equal altitudes and are situated nearby.This is less true for Paranal and Armazoni. With significant differences regard­ing both altitude and coastal distance as weH as pattern of neighboring landscape,equality of integrated atmospheric water vapour cannot be taken for granted. Atthe same time, presumptions are not necessary, since a large amount of com­parison data exists, obtained over several years. Figure 5.19 shows the result ofsuch comparisons between Paranal and Armazoni. With all data expressed interms of amount of precipitable atmospheric water vapour, the difference be­tween the data obtained at Paranal and Armazoni has been plotted versus thosederived for the summit of Paranal. We conclude that no significant difference inintegrated content of water vapour can be established between the atmospheresabove Paranal and Armazoni.References: see Chapter 2

CHAPTER 5. ANALYSIS122

50

40

10

___I

,--- ---,I I, .I II I, I

'---I,I

12102o 4 e BWATER VAPOUR CONTENT (mm)

Figure 5.12: Same as Figure 5.11 but for nights during spring and fall. Spring and falltimes are defined as from Getober 1 to November 30 and from April 1 to May 31,respectively, all limits inclusive.

50

40

ii.... 30zoH

t~u. 20

10

j---I,I,I

122o 4 B B 10WATER VAPOUR CONTENT (mm)

Figure 5.13: Same as Figure 5.11 but for sumOlCr nights. Summer time is defincd aslasting from December 1 to March 31, both limits indusive.

CIIAPTER 5. ANALYSIS

50

40

123

M-30zoH

~ 20

10

o

IIIII__ .1

2

---,---I

IIII

•I

4 B B 10WATER VAPOUR CONTENT (mml

12

150

"0

Figure 5.14: Same as Figure 5.11 hut for daytime.

10

o

,--I,IIII___ I

2

---IIIIIII--- ...

I1 ,

III,I11I

4 B B 10WATER VAPOUR CONTENT (mm)

12

Figure 5.15: Same as Figure 5.12 hut for daytime.

CHAPTER 5. ANALYSIS 124

50

--- .., ,, I

: I____ I

III--- ..----• J

I,,---.

40

o 2 4 e e 10 12WATER VAPOUR CONTENT ImmJ

Figure 5.16: Same as Figure 5.13 but for daytime.

12

10

2

II

I A II \ /I I

1 \ 1\/ 11\ 1V

...

~, ,

, I f\, I 1 \ I,

1 1 1

~J\/_ \//\'-v!.JW·

O'";;-.L...-;;:":-...L.-;:-!:--.L...~:-.L--:-l--:--I--IL:-L-..l-..L.....L-...J..J0.3 0.5 0.7 0.9 LI L3 L5 L7

FRACTION OF YEAR

Figure 5.17: Comparison of seasonal variations of average periodical amounts of inte­grated water vapour in the atmospheres above La Silla and Paranal. Observed amountsof precipitable water vapour, expressed in millimetres, are plotted versus observingepoch. The graph covers a time interval of 18 months, and averaging has been madefor periods of te~ days, accumulated from 1983 to 1989. Data for La Silla are indi­cated by a dashed curve, those for Paranal by a full-drawn curve. All data refer toobservations made during nighttime.

CHAPTER 5. ANALYSIS 125

---r-,--.---,-y---,-,

'l12

10

I! "1\

ffi 8 ' II I

~ 1\ '\,

g 8 1 ",

I'1 ,\ I ,

/'1> 1 ,ffi .. " \.. • I I 'I..

I \:&

I

2

~0 .L....I.-_J._.J....--I_--1--l..-L-.L--L..J....--1-L-..L.-J.J0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

fAACTION Of YEAA

Figure 5.18: Same as Figure 5.17 but referring to observations made during daytime.

PARANAL - ARMAZONI3

2..'

, , ,1 , , ,

UJ " r ".' , 'u • • • • • I·ffi ~... f \" ,-; , • • • •"~""~\i\l .: . ," ."a: 0 • • • • •UJ .~ •IL. 'a •••• • • • '.IL. •

H • ~c .,-1 • .-.,'. • ••-2

-30 1 2 3 4 5 B 7 8 9 10

WATER VAPOUR CONTENT PARANAL

Figure 5.19: Comparison of amounts of integrated water vapour in the atmospheresabove Paranal and Armazoni. The differences in precipitable water vapour, expressedin millimetres, between Paranal and Armazoni are plotted versus the data obtained atParanal. Measurements at the two summits are simultaneous, refer to both nighttimeand daytime and cover several years.

CHAPTER 5. ANALYSIS

5.3 Meteorology

126

5.3.1 Relative humidity

The western part of the eentral Andes area is unmatehed in the Southern hemi­sphere for low precipitation and humidity. While La Silla lies at the southernedge of the dry area, Paranal is in its eentral part. The distributions of nighttime1 hour averages are eompared in Fig. 5.20

The "wet" season at Paranal lasts from January to April with an averagerelative humidity of 20% eompared to 5% (6% in 85) during the driest month(September). Condensing humidity, freezing rain or fog may nevertheless appear,however for no more than a few days per year, mainly in April (Bolivian winter)and July-August. Snowfalls oeeur roughly onee every other year and the snownever lasts more than one day with a thiekness of a few ems only.

The relative humidity at La Silla presents smoother transition periods. The"wet" season lasts there from Deeember to February with 43% average relativehumidity eompared to 17% (12% in 85) during the driest month (July). Con­densing humidity may oeeur at any time during the wet season. Fog, freezingrain and snowfall ean oeeur in winter, though the snow seldom lasts more thana week, with a thickness of less than 50em.

5.3.2 Temperature

The 24h eumulative plots of the whole yearly set of Ih average temperaturesampIes show a well defined diurnal eycle for eaeh site. The time referenee is thetrue solar time, or loeal apparent time. A fit with 6th order polynomials is madefor eomparison of annual eharacteristies.

Both resultant eycles are presented on Fig. 5.21, error bars eorrespond to ±the standard deviation of the fitting proeess. Beeause of a smaller seasonal trend,Paranal data have a smaller dispersion (rms 2.7C) than at La Silla (rms 4.1C).On the other hand, noeturnal temperature eyc1es are identieal on both sites withan average amplitude of 1.1C.

On the same graph, the lower eurves show the annual average diurnal eyclesof the temperature differenee between ground and air. The ground temperaturesensor is loeated under a lOcm cover of typical soil. The ground at Paranal hasa smaller thermal constant than at La Silla.

The monthly average of the noeturnal temperature variation presented inFig. 5.22 is an indieation of the temperature stability of the site during the courseof a night. It remains lower than 5C at night with, again, a larger seasonal effectfor La Silla.

CHAPTER 5. ANALYSIS

(l)

<Si~

Paranal

&

~...~..~.. (l)

a csiIn

%

gu

cg

viN

Relative Humidity, 1986

513. e 75.13 11313.13HUM IDI TY (PERCENT )

127

Figure 5.20: Cumulative probability of ground level nighttime average relative humidityin 1986 at Paranal (dashes) and La Silla (solid line).

20.1310.0 15.13SOLTIM

5.0

: ,

il! ..•• • J.

(l)

vi

-u:..~ :

Figure 5.21: Polynomial fit of diurnal average cycles at La Silla (solid line) and Paranal(dashes), for air temperature (upper curves), and ground to air temperature difference(lower curves).

CHAPTER 5. ANALYSIS 128

(Sl

.;

...(Sl.

U-M

0LU:>a:

(Sl

N

.........

: : .

P.il-r.a,Qil-J..........

: ~.......J_an-,-F(_!b......M_a_rI-A..:...pr......M-.:ay.I-J_ul-L1_Ju_l'-A.....:ug::.J.S,;,..:c..:...PL.:0..::.ct~N o::...:.v",;:D:....:e..:L-.....J

MQNTH

Figure 5.22: Monthly average of differences between nocturnal air temperature extremaon La Silla (solid line) and Paranal (dashes).

5.3.3 Wind

As shown on Fig. 5.23, the night wind average velocity was above lOm/s during19% of the time at La Silla(10% in 85) and 26% on Paranal(20% in 85). Themain difference between the two sites lies in the period January to March duringwhich La Silla benefits from very mild conditions. This explains also that theamplitude of the seasonal variation is thus lower on Paranal (3m/s) than at LaSilla (5m/s). The number of days with possible occurrences of wind velocitieshigher than 20m/s is higher at Paranal (50/year) than at La Silla (25/year).

Polynomial fits of diurnal cycles of wind speed are plotted in Fig. 5.24 forthe maximum and average wind speed during each 20mn sampie of the wholeyear 1986. Both sites present a maximum during the afternoon and the Paranaldiurnal cycle has a larger amplitude.

As can be seen in Fig. 4.54 and Fig. 4.56, each site presents one predominantdireetion for high velocity winds with a smaller secondary maximum nearly fromthe opposite direction. Paranal stands eloser to the sea shore and is more affectedby thermal winds during daytime.

The turbulence intensity is a non-dimensional parameter defined as the rmsof velocity fluetuations during each 20mn averaging period normalized by theaverage velocity . Its analysis confirms that nighttime is less turbulent thandaytime for botH sites. Both sites are within the usual tolerances for construetion(the common values assumed for the effect of wind on buildings are between 0.1and 0.2).

CHAPTER 5. ANALYSIS 129

25.0'

.......'...........

/:/P.~Mi\1

~.(:/ :':'1

",

.":..L:

: ."":..: ~

" ",

··i

Cl)

Gi~......L.-'-~~-L.-'-.l....J':-l:~...l-L.J--L~...l-L.J-l....l.-.L...L.J.-L...J0.0 20.0

es>vi'"~

wl:I-

~I- es>:r GiCl InZ

5l:::>u

es>viIV

Figure 5.23: Statistical distribution of nighttime wind speed at La Silla (fullline) andParanal (dashes) .

Wind .pud diurul cyele: O=Max., O=Ave.

Paranal .... - .-LaSilla __

10.0. 15.0 20.0SOll IM IH)

Cl)

lSl L...J'--J.--'---L.....L-...l.-..I-l-I'--J.-I-~....l.--'-..I-l-J--l.-I-..L-'-..L-.l-L...J._

Figure 5.24: Diurnal cycle of average (0) and maximal(O) wind speed as a function oftrue solar time on La Silla (solid line) and Paranal (dashes).

CHAPTER 5. ANALYSIS

5.4 Seeing

130

5.4.1 Accuracy of the measurements

Seeing is expressed in arcsec as the Full Width Half Maximum of a stellar imageat the focus of a telescope limited by the atmosphere at 0.5j.Lm and pointingtowards the zenith.

The output of the Seeing Monitor consists of statistical estimates out of 200short exposures. Measurements are taken from the differential motion in thedireetions parallel and perpendicular to the pupil separation, which gives twostatistical estimates of the instantaneous seeing.

The measurement accuracy is conditioned by:

• intrinsic statistical error which, for 200 exposures, amounts to ±6% on eachdireetion of measurement, 01' ±4.25% on the average of both.

• uncertainty in the estimate of the instrumental noise: aglobai figure is usedfor the instrumental noise according to laboratory measurements simulatinga flat wavefront (seeing=O). This figure is subtracted from the measured rmsimage motion before conversion into FWHM. The nominal relative error ineach direetion of measurement after subtraetion of the instrumental noiseis larger for bettel' seeing (negligible above 1 arcsec seeing, ±1% for a 0.5arcsec seeing, 01' ±5% for a 0.2 arcsec seeing)[2].

Adeparture from this reference value may be caused by misalignments aftertransportation, long term degradation of optical surfaces, accumulation ofdust in the optical path, excessive mechanical vibrations of the telescopemount 01' tower, 01' large tracking errors inducing rapid drift of images inthe focal plane, as described in seetion 5.4.2. In such a case, the instru­ment systematically overestimates the seeing because of an increase of theinstrumental noise in terms of apparent rms motion. This is why, in thecourse of the site survey, all technical modifications 01' software updateswere implemented at the same time on all monitors. One can then get ridof instrumental zero point errors by comparing the difference of differentialmotions measured at each sites.

• exposure time averaging, ie: when turbulent layers move at fast speed withthe wind, the lOms exposure time is too long to freeze completely theatmosphere and the tilt of the wavefront is underestimated. Such eventsare easily detected by the DIMMs because longitudinal and lateral relativemotions are not attenuated by the same amount and lead to a disagreementin the value of the equivalent seeing. It is possible to evaluate an averagespeed and t~us to compute a correction coefficient using [1] where the wholeturbulence is assumed to be concentrated in a single layer. But because ofthe pOOl' accuracy of this method and considering the small amount of data

CHAPTER 5. ANALYSIS 131

concerned in Chilean sites, the correetion for high frequency motion has notbeen included for this site comparison.

• the pixel angular scale is the only instrument dependent calibration pa­rameter and enters into the conversion of image motion to seeing at thepower 1.2 . It is regularly checked by direet imaging of an optical doublestar of known separation (the typical value for our DIMMs at F/15 is 0.87arcsec/pixel). The accuracy of the calibration method is bettel' than 1%.

The overall nominal error budget of a DIMM, excluding possible underestima­tion due to exposure time attenuation, goes from ±10.5% (4.25+5+1.2) with aseeing 0.2 arcsec to less than ±6.5% (4.25+1+1.2) above 0.5 arcsec. The accuracyis even bettel' on long term averages because of the decrease of the statisticalnOlse.

5.4.2 Quality of the data base

Measurements started first at Paranal and then at Vizcachas while the DIMMconcept was still under development. Seeing statistics presented in Chapter 4.5cover the period from Oetober 1988 to August 1990. The data of the first 11months are not at the nominal accuracy because in September 1989, a unfore­seen instrumental noise linked to the pOOl' tracking performances of the DIMMtelescopes was detected and eliminated:It takes about 60 seconds to acquire the 200 exposures necessary for determiningthe current seeing. If during that time, the star drifts exceedingly in the fieldof the telescope, field aberrations in the imaging doublet modify the apparentseparation of the two spots on the CCD. Large periodic tracking errors, whichmay cause the star to cross the whole field of view during one measurement,produce an apparent differential motion of the same order of magnitude as theone due to the atmosphere.

The consequence is an additional noise not correlated with the site seeingconditions and constant over large enough periods of time (lhour). Its magnitudemay be estimated as explained in 5.4.8 where the whole data base is used forthe analysis of seasonal trends. Nevertheless, for an accurate and unambiguouscomparison of the candidate sites, the data base was restricted to measurementstaken after the DIMMs had reached their nominal accuracy (ie: after September1989).

References[1] H.M. Martinj Image motion as a measure 0/ seeing qualitYj PASP 99, 1360,Dec. 1987.[2] M. Sarazin, F. Roddier The E.S.O Differential Image Motion Monitor; As­tron. Astrophys. 227, 294-300 (1990).

CHAPTER 5. ANALYSIS 132

5.4.3 Comparison La Silla-Vizcachas

On several occasions, it has been possible to verify that La Silla and Vizcachashave the same seeing quality, either by comparing DIMM measurements at Viz­cachas with NTT (New Technology Telescope) data, or by using two DIMMssimultaneously. In the latter case, the La Silla DIMM was located elose to theSchmidt telescope, away from any man made structure in the direction of theprevailing wind.

5.4.4 Comparison Paranal-Vizcachas

-Seeing: With regards to the statistics of houdy averages summarized in Ta­ble 5.2, both sites are quite comparable, with an advantage of 15% for medianseeing at Paranal.

Figure 5.25 shows the cumulative probability of one hour average seeing at500nm wavelength at Paranal and Vizcachas for the common period of mea­surements after the DIMMs reached their nominal accuracy (Sep. 89 to Aug.90).

Table 5.2: Statistics of 1 hour average seeing for the period September 1, 1989 toAugust 31, 1990.

Month Min 5% Max 95% Ave 50% Sigma nr.hoursVizcachas 0.29 0.48 2.72 1.28 0.81 0.76 0.26 1818Paranal 0.27 0.42 2.45 1.17 0.71 0.66 0.24 2779

-Scintillation: The index of low frequency « 50Hz) scintillation, extractedfrom the same set of data and presented in Table 5.3 shows a difference of 25% infavour of Paranal, after correction of the raw data with the cross-calibration coef­ficients given in section 4.6. As shown in Fig. 5.26 the worst conditions appeared

Table 5.3: Statistics of 1 hour averages of the index of low frequency scintilla­tion for the period September 1, 1989 to August 31, 1990.

Month Min 5% Max 95% Ave 50% Sigma nr.dataVizcachas 0.009 0.019 0.35 0.122 0.052 0.042 0.034 1818Paranal 0.006 0.016 0.16 0.079 0.038 0.033 0.021 2979

on both sites during winter of 1989 but the seasonal cyele did not repeat in 1990.The correlation ~ith the wind velocity at the tropopause is not systematic, asalready seen on Fig. 4.70 (section 4.6).

CHAPTER 5. ANALYSIS 133

The low frequency part of the scintillation is not a constant fraction of thetotal scintillation and must be used only qualitatively. The total (500Hz) indexof scintillation was also measured at Vizcachas and an average of 0.132 wasfound (on a total of 1350 hours) i.e. 2.5 times larger than the average obtainedfor simultaneous measurements at low frequency (Table 5.3).

5.4.5 Comparison La Montura-Paranal

Figure 5.27 shows the cumulative probability of houdy average seeing at 500nmat La Montura and Paranal for the simultaneous measurements (Apr. 89-0ct. 89,instrumental noise included until Sep.89). La Montura is able to offer periods ofexcellent seeing which are disrupted by sudden deteriorations which sometimeslast several hours. The effect of bad seeing periods is noticeable when comparingthe average seeing given in Table 5.4 and standard deviations. A comparison of

Table 5.4: Statistics of 1 hour average seeing for the period April 1, 1989 to Oetober31, 1989 at La Montura and Paranal (data include instrumental noise until Sep. 89).

Month Min 5% Max 95% Ave 50% Sigma nr.dataParanal 0.29 0.55 2.37 1.39 0.89 0.85 0.27 1394

La Montura 0.36 0.52 2.85 1.87 1.00 0.88 0.43 1394

microthermal measurements at the top of the towers shows that La Montura ismuch more affected by local turbulence than Paranal (measured at 5m height,the average Cl = 0.003](2.m-2/3 at Paranal versus0.012](2.m-2/3 at La Montura). This means that a telescope would have to bebuilt on a much higher platform at La Montura to benefit from the same condi­tions as at 5m height at Paranal.

5.4.6 Comparison Armazoni-Paranal

DIMM3 was operated on Cerro Armazoni on a 2m high platform. Measurementsstarted at Armazoni first at the very top of the summit (location 1) from Febru­ary 11 to March 11, 1990. The telescope was then moved 177m upwind to thenorthern end of the summit (location 2). The two sites, Paranal and Armazoniare hardly distinguishable on the statistics of Table 5.5. The slight difference infavour of Paranal is due to the measurements at location 1 (214 hours) where 10­cal effects were not completely negligible at 2m height above ground. As shown insection 4.7, the nighttime turbulence in the 2m-5m layer corresponds to 0.1 arc­sec equivalent FWHM. On the other hand, during the measurements at location2 (151 hours), the two sites differed by less than 1 percent.

CHAPTER 5. ANALYSIS 134

--',,-

September89 to August90

One hour seelng averages

,,/

I,I

(

II

II

II

I

Seelng 10 er/than 0.5arcsec/Paranal=450h/2779h=16.2x•• / •••• 0 ••••••• /Vlzcachas=124h/1818h: 6.8%

II

I

I Median Paranal=0.66 arcsecI

I ." •• Vlzcachas=O. 76 arcsecI

I/

,/

.2

o

.8

~.4coE:JU

~

ClE

....

...;.6oUcoI-....

o .5 1fave (arcsecl

1.5 2

Figure 5.25: Cumulative probability of 1 hour seeing averages from September 89 toAugust 90 at Paranal (fullline) and Vizcachas (broken line) given in fraction of totalobserved time.

. 12 r-1,.,~-,.....,--,---r..,-....--r-"-r-1r-r-,--,--r--r-,-....--r-...-,.....,~--,---r--,-,....-,

1989 1990

/ '\,\ "\

II

I

. 1

-~ .08

.Jc

b!. 06

I-0u

1C.~ .04

.02

80

10 24 ~ ~

MONTH

Figure 5.26: Com~arison of monthly averages of the index of low frequency scintillationfrom September 88 to September 90 at Paranal (fullline) and at Vizcachas (brokenline).

CHAPTER 5. ANALYSIS 135

100 Table: binX m

75

'VE:;:;~

50c.;;S:ltJ

25

.5 1fave

1.5 2

Figure 5.27: Cumulative probability of nightly seeing averages (20mn bins) from Apr.89 to Oct. 89 at Paranal (solid line) and at La Montura (dotted line). Data includeinstrumental noise until Sep. 89.

Table 5.5: Statistics of 1 hour average seeing for the period February 11, 1990 toMarch 31, 1990 at Armazoni (2 locations included) and Paranal.

Month Min 5% Max 95% Ave 50% Sigma nr.dataParanal 0.30 0.44 2.22 1.08 0.68 0.63 0.22 365

Armazoni 0.29 0.44 2.19 1.11 0.70 0.65 0.24 365

CHAPTER 5. ANALYSIS 136

5.4.7 Comparison Vizcachas-Las Campanas

Two identical differential image motion seeing monitors have been used to com­pare seeing conditions at Las Campanas observatory and at Cerro Vizcachas. Itis shown that the two sites, because of their proximity, behave similarly and thata large part of the turbulence conserves itself while carried by the wind.

In the frame of the site evaluation for the VLT, it appeared of interest to theSite Selection Working Group to compare one of our potential candidate sites toan existing observatory in Northern Chile, other than La Silla. The observatoryof Las Campanas was chosen, where a similar analysis was conducted by theCarnegie Institution in preparation for the construetion of an eight meter dasstelescope.

Thanks to the good will and cooperation of local administrations, it was possi­ble to run several instruments simultaneously on both sites during 14 consecutivenights, from November 27 to December 10, 1989.

• The observatory of Las Campanas is composed of several summits alignedalong a large ridge roughly facing the prevailing North wind, standing weHabove the surrounding terrain. Our test point was on the western part,elose to the Dupont telescope at 2280m altitude. A mobile differential imagemotion monitor (DIMM3), identical to DIMM2, was operated on a 2m highplatform at the northern ridge of the summit. Wind speed and direetion atground level are recorded online. .DIMM3 was located elose to the Carnegie seeing monitor [1], itself installedon a 5m high platform. Because of the terrain dedivity, the height differencebetween the two monitors was smaller than 2m. The Carnegie monitordelivers the average seeing over 500 second periods.

• Cerro Vizcachas: the Vizcachas/La Silla ridge stands at 2400m altitude,27km South of Las Campanas, separated from it by a rather perturbedlandscape ineluding several mountain ridges of nearly the same altitudeand 1000m deep canyons. The differential image motion seeing monitor(DIMM2) at Vizcachas is operated on a 5m high platform, elose to theupwind slope. An acoustic sounder (Sodar) is located at the northern footof the seeing tower, 10m under the seeing monitor level.

-Results: The 14 days campaign lasted from 27 November at 3h U.T to 12December at 9h U.T, and produced more than 3000 individual measurements oneach site. Statistics of those raw data are presented in Table 5.6. The correspond­ing cumulative pI,'obabilities are plotted on Fig. 5.28. The 1mn seeing estimateswere binned every 500 seconds for compatibility with the Carnegie monitor.

As can be seen on Fig. 5.29, the simultaneous time series of 500 second seeingbins are very comparable (the horizontal axis represents the sequence numberregardless of time intervals between measurements).

CHAPTER 5. ANALYSIS 137

Table 5.6: Statistics of 1mn average seeing for the period November 27 to December10, 1989 at Las Campanas and Vizcachas.

Site Min 5% Max 95% Ave 50% Sigma nr.data nr. nightsCampanas 0.27 0.38 1.44 0.86 0.60 0.58 0.16 3,727 14Vizcachas 0.28 0.36 2.21 0.91 0.60 0.57 0.20 3,520 14

1.5

----

.5 1FWHM (ARCSEC )

I'I'

n l

1

::~I,11

~ilIUII1

~(11JI,1

~

o

o I------"'=-~

20

80

40

60

100

Figure 5.28: Cumulative probability of raw seeing data at Vizcachas (dotted line) andLas Campanas (fuUline).

CHAPTER 5. ANALYSIS 138

.5

1 - !

1~~\J~'A!\!lvV~~~lJ~~~f\j400100o 200 300

SEQUENCE

Figure 5.29: Simultaneous seeing measurements at Vizcachas (fullline) and Las Cam­panas (dotted line). The Vizcachas data are offset by +1 arcsec for clarity. The hori­zontal axis represents the sequence number regardless of time intervals between mea­surements.

-Analysis of the temporal correlation: In the theory of atmospheric tur­bulence one often uses the hypothesis that the turbulence moves with the windwithout modification of its statistical properties along the path. This is perfectlyverified on short distances (100m) and used for instance in the determinationof the temperature structure coefficient with only one sensor as described inSection 3.6.

As the two tested sites are 27km apart along the north direction, this data setmay be used to check that part of the turbulence detected at Las Campanas istransported without much changes to Vizcachas.

Fig. 5.30 shows that the normalized temporal intercorrelation of the two timeseries presents a weH defined peak corresponding to a 50 minute delay of Viz­cachas with regards to Las Campanas. On the same graph, the autocorrelationfunetions of the seeing at the two sites show the time constant of the atmosphere.The similarity of the autocorrelation and intercorrelation peaks induces that theturbulence was transported at a relatively low dispersion velocity of gm/so Thelevel of the intercorrelation peak corresponds to 40% of the produet of the rmsseeing on the si~es.

This assumption cannot be verified because of the lack of information on thewind velocity in the first 1000m above the sites, but it gives great hopes for theefficiency of seeing predietion of the future V.L.T seeing monitoring station tobe used for flexible scheduling. A seeing monitor placed upwind a few tens of

CHAPTER 5. ANALYSIS 139

180120-120oJ......L,....L.-.L-J-....L.-.L-J--'-'-.L-J--1-.L-J--1-'-'--'--L-J...-l.-L-J.......L.-J'-L....L.-J,.;"J.......L.-J---I.....ll-l.....l. •..l-J

-180

-N:ES:a':::".5

1-01-0o~CIi......S

-60 0 60lagtime (minutes)

Figure 5.30: Temporal correlation of 500 seconds seeing bins : intercorrelation (fullUne), Vizcachas autocorrelation (dashed Une) and Campanas autocorrelation (dottedUne).

kilometers in the direction of the prevailing wind, associated to a Doppler 3Dacoustic sounder would permit a typical one hour warning on changes in seeingconditions. It is to be recalled that nearly 80% of the total seeing on our chileansites is contained in the 50-1000m layer.

7"Comparison with the Carnegie seeing monitor: If one discards highwind speeds, the seeing measured by the Carnegie monitor elose to the DIMMwas weIl correlated with the DIMM measurements after systematic corrections ofthe Carnegie monitor raw data (by a factor of approximately +20%) to accountfor the transfer of frequency components through the electronics as weH as forslow motion image wander (frequency below 2Hz) as explained in [1].

Acknowledgments: We are indebted to the crew of Las Campanas Obser­vatory for their warm hospitality and in particular to E. Cerda, operator of theCarnegie monitor.

References[1] S. E. Persson, D. M. Carr, and J. H. Jacobs. Las Campanas ObservatorySeeing Measurements; Experimental astronomy, Vol.1 no : 3, 1990.

CHAPTER 5. ANALYSIS 140

5.4.8 Seasonal variations of seeing

It may be shown that the data taken before September 1989 can still be veryuseful for qualitative analyses of the correlation of seeing with meteorologicalparameters. Because it is caused by tracking errors, a parameter independent ofatmospheric conditions, the instrumental noise may be partly removed a poste­non.

This noise being additive in terms of variance, its average value can be esti­mated comparing the median seeing over several months before and after Septem­ber 1989, on the same site, assuming that year to year variations are negligible.The term to subtraet is weighted in such a way that the corrected data have thesame statistical distribution as the uncontaminated ones; if (7~ is the noise termfound by comparing medians of the first and second periods:

(7~ = Median1~ - Median2~ (5.1)

(5.2)

The subtraetion is done in such a way that the correeted set has the same lowerlimits as the reference set. One possible solution is a linear weighting of the noiseterm with the distance to the minimum:

FWHMc

= [FWHM1 ~ _ (72 FWHM1 - Min2] in Median1 - Min2

Following this law, all available data of the first period have been corrected andappended to the uncontaminated set for the analysis of the seasonal variationof seeing conditions on the sites. The noise terms computed for Vizcachas andParanal on 11 months of each set differ by less than 10% and the rms of thedifference between the reference and corrected statistical distributions is 0.01arcsec.

The results of the analysis of seasonal trends is shown on Fig. 5.31 where fits ofthe median seeing on 5 days segments were obtained with all the available data,ie: out of 4167 hours at Vizcachas (Oct88-Aug90) and 7884 hours at Paranal(Oct87-Aug90). Two main meteorological perturbations, the so-called Bolivianwinter (January-February) and the normal Chilean winter (July to September),are visible for both sites, but with a much larger amplitude at Vizcachas.

CHAPTER 5. ANALYSIS

Table: fitmedIan

141

AUTUMN WINTER AUTUMN

.9

~.8UlUL.CO

0-

5S.... 7.:

.6

. 5 l---l._-L-_-L_..l-_l----l._-.l-_-L_-'--_'----l._-'-_......I-_-'-----'

.25 .5 .75 1 1.25 1.5fractlon (of.yearl

Figure 5.31: Seasonal variation of 5 days median FWHM at O.5JLm and at zenithmeasured 5m above ground on Vizcachas (upper curve) and Paranal (lower curve).Peak corresponds to Chilean winters, secondary peak is linked to Bolivian winter.

CHAPTER 5. ANALYSIS

5.5 Other parameters

142

5.5.1 Airglow

According to geophysicists working in this field, so far no systematic night air­glow measurements have been made anywhere in Chile. Because of the strongtime variability (on time scales between seconds and decades) meaningful directmeasurements would have to cover aperiod of at least ten years. On the otherhand, some basic conelusions on the differential airglow aetivity at La Silla andParanal can be drawn from the known generallatitude dependence of the meanairglow intensity (cf. [1] and [2]), from measurements in other South Americancountries ([3], [4], [5]), from measurements from ship and aircraft over the SouthAtlantic, and from satellite data (quoted in [1]). According to these sources nei­ther La Silla nor Paranal is located in or elose to a region of enhanced averageairglow activity. Neither site is expeeted to show an increased night airglow in­tensity due to the South Atlantic magnetic anomaly. The essentially flat relationbetween airglow intensity and latitude typical for the latitude range in questionsuggests a time-averaged intensity difference of less than 10% between La Sillaand Paranal.

Henee, aceording to the information available in the geophysicalliterature eon­eerning the night airglow intensity, there seems to be no significant differeneebetween La Silla and Paranal.

References:[1] Silverman, S.M.: 1970, Space Seienee Rev. 11, 341.[2] Chamberlain, J.W.: 1961 Physics of the aurora and airglow. Academic Press,New York.[3] Sahai, Y., Bitteneourt, J.A., Teixeira, N.R., Takahashi, H.: 1983 Ann. Geo­physicae 1,291.[4] Sahai et al.: 1988, Planet. Space Sei. 36, 371.(5] Takahashi et al.: 1989, Planet. Space Sei. 37, 649.

5.5.2 Thermographie survey

In Deeember 1989 a thermographie eomparison of potential VLT sites was un­dertaken both from the ground as weIl as from an aireraft using an Inframetrics10pm infrared video eamera. Observations were done at night. None of the sitesexamined showed any disturbing thermal signature. In eontrast to the alreadydeveloped sites (La Silla, CTIO) they were almost indistinguishable from thesurrounding landscape indicating them to be part of a homogeneous thermal en­vironment. In c~mtrast, the developed sites are severely thermally polluted bothbeeause of the hbavy eonstruction on the sites (eg extensive asphalt and eoneretepaving being heated by the sun in daytime) and by the heating of buildings onthe site. This must have a deteriorating effeet on the atmospheric seeing of the

CHAPTER 5. ANALYSIS 143

site. In the development of the VLT site such thermal pollution should be mini­mized in order to preserve the "pristine" thermal conditions of the mountain topas it exists now.

An interesting condition was noted for Cerro Paranal. The cone of this moun­tain has an asymmetrical temperature profile with the westward (seaward) di­rection being warmer by about 1 K than the eastern (landward) direction. Thisprobably results from a combination of the seaward side facing the (always, atnight) warm ocean and from it being sloped so that it views more land mass andless cold sky. It is clearly not the result of residual solar heating of the westwardside, since the temperature of the mountain site rapidly settled to nighttimeconditions after sunset. Seeing measurements at Paranal do not show that thiscauses major problems.

5.5.3 Light pollution

All the site candidates in Northern Chile subjected to detailed investigation ofsite quality parameters benefit from the absence of disturbing artificial lightpollution. This concerns stationary as well as variable sources of light pollution.None of the sites in question have a direet line of sight to any light pollutionsource of importance.

Regarding the site candidates of highest interest, La Silla, Vizcachas, Paranaland Armazoni, there are some differences in the low levels of artificiallight pol­lution present. In the case of La Silla and Vizcachas, horizontal light pollutionfrom La Serena and Coquimbo is obvious although quite faint and affeeting onlya small part of the sky immediately above the horizon. Similar effects exist inthe direction of Vallenar and Domeyko. Finally, some light pollution is notedfrom the Panamerican highway. Whilst further growth of La Sercna and Co­quimbo seems evident, there is hope that this will be accompanied by attemptsat control of light pollution of the sky by city authorities.

Both Paranal and Armazoni are virtually totally void of any problems con­nected with artificiallight pollution. The only significant pollution centre present,common to both places, is the city of Antofagasta. The effect is only indirect andvery faint, weIl below that of La Serena and Coquimbo on La Silla and Vizcachas.Even with a significant increase in the illumination of Antofagasta, Paranal andArmazoni would remain sites of uncommonly low artificial light pollution.

5.5.4 Seismicity

- Seismic hazard (see Appendix B).Several studies from different specialized institutes, analyzed in Appendix B, donot show any significant difference between the Paranal and La Silla areas as faras seismic hazard for construetion is concerned. An analysis of the geology of theParanal mountain itself is underway.

CHAPTER 5. ANALYSIS 144

- Microseismieity (by Ph. Boudon, Cerga Obs.).The level of microseismic aetivity on the VLT site is of importance for interfer­ometric observations. Precision accelerometer observations were therefore madein March 1989 on Paranal, La Montura, and Vizcachas. Sensors were placed onthe surface of the ground, mostly in the North/South direction. The frequencyband covered 1 to 200 Hz.

We focus here on a comparison between Paranal and Vizcachas which is basedon measurements made on Paranal on 6 and 7 March (night and morning j windo to 5 m/s) and on Vizcachas on 12 March (afternoon; wind 5 to 10 m/s).The accuracy is limited by sensor noise, power supply stability and surface noise(wind, power generator and other human local activity).

The measurements made give a coarse idea of what to expect on the stationarylevel of seismic aetivity. For a more accurate picture these measurements shouldbe correlated with the seismic aetivity in Chile on a long time scale. Analysis showthat the measured seismic vibrations are significantly smaller than the vibrationscaused by telescopes, their attachments, or by other equipment likely to be usedin interferometers. The data do not show any significant differences between bothsites except, perhaps, on surface transmission and damping j information on topsoil transmission and damping cannot be reliably extrapolated to the behaviourof concrete foundations.

Seismic jolts in the measurements were difficult (if not impossible) to separatefrom human or other natural interferences aeting on the 'surface of the ground.

5.6 Summary

At this point, it appears already that the Paranal area is astronomically superiorto the La Silla area on the following points:

• Number of photometrie hours (cloud cover).

• Relative humidity.

• Precipitable water vapour (and/or sky emissivity).

• Seeing.

• Light pollution..

• Wind speed at the tropopause.

On the other hapd, the two areas are equivalent with regard to the other pa­rameters monitored, with the exception of the higher wind velocity at Paranal.Quantitative figures will be given, together with figures of merit, in Chapter 6.

Chapter 6

FIGURES OF MERIT

6.1 Introduction

Site quality cannot be expressed by a single figure because site users belong toa heterogeneous community. Yet, the cost of building the VLT on one or theother site will come out as one figure which has to be related in some way to theexpeetations of the astronomical community.

The working group tried in what follows to go a little further than a purelyqualitative assessment of the sites, and thus decided to attempt to quantify thepotential scientific output of the VLT by relating observing modes (possiblyrelated to the fields of expertise of its members) to site parameters using verysimple, although representative, power laws.

The reference criterion of merit was defined as the necessary exposure timeto reach a given signal to noise ratio when observing a given object with ahypothetical, nearly perfeet, VLT and its instrumentation, on one of the testedsites. It is therefore a "quantitative", more than a "qualitative" approach that isconsidered. After identifying the relevant site parameters, the group members de­termined at which power the parameters were to be taken into account accordingto the way the telescope was going to be used.

Moreover, because nobody can presently foresee the relative importance ofdifferent types of observations in the year 2000, it was agreed not to mix thevarious figures of merit into one single coefficient, but rather to compare sitesindividually for each mode.

With regard to the observational parameters, the following classification wasagreed upon:

• Examples of representative observing modes: Direet imaging, Spec­troscopy and Interferometry.

• Wavelength was split into five representative bands charaeterized by theircentral wavelength: Visible ( " = 0.55p.m (V)), near Infrared (" = 2.2p.m

145

CIIAPTER 6. FIGURES OF MERIT 146

(K)) , Infrared ( ,\ = 4.8pm (M),,\ = lOpm (N), ,\ = 20pm (Q)). Severalbands ean be merged in many eases. M and Qwere introdueed to study theinfluenee of sky transpareney on the eoeffieient of merit in two regimes, Mfor the seeing-limited situation and Q for the diffraetion-limited situation.

• Types of objects were divided into two types: extended and point like.Extended objeets are defined as "truly extended" for whieh no high angularresolution is required while point like objeets include extended objects madeof several point sourees or fine struetures (filaments ete... )

• The spectral resolution ean be low or high (L/H).

The diseussion on the ehoice of the relevant site parameters is presented inseetion 6.2, followed by the ealculation of power relations for eaeh observingmode in seetion 6.3. The final results are given in seetion 6.4 where the relativemerit of Paranal versus La Silla is eomputed wherever statisties on the parametersinvolved are available.

6.2 Selection of site parameters

6.2.1 Introduction

The parameters used in the analysis are the following:

• Photometrie nights: always welcome by the astronomers ('Y' for yes in thetables)

• Speetroseopie nights: when speetroseopie, but non photometrie, eonditionsare not suffieient, a 'no' appears in the tables.

• Water vapour: related to sky emissivity as explained in seetion 6.3.

• Transmission: though not measured, may be related to sky emissivity insome eases as explained in seetion 6.3.

• Seeing: median values in aresee.

• Speekle lifetime: though not measured, may be related to seeing as explainedin seetion 6.2.3.

• One parameter whieh was not eonsidered for the evaluation of the figures ofmerit is the wind, sinee it is considered more as an "engineering" parameter.It will however appear in the determination of the figure of exeellenee (seeseetion 6.5).

We use statisties eomputed on the largest available period for eaeh parameter.

CHAPTER 6. FIGURES OF MERIT 147

6.2.2 Site requirements for infrared

Infrared operation is specially demanding on both telescope cleanliness and at­mospheric properties. The constraints that are easy to list for the whole infraredrange are photometric quality in infrared bands, good seeing, low air tempera­ture. In the near infrared (below 2.5 J.lm), the sky non-thermal brightness must beadded as being responsible for the sensitivity limit outside the thermal domain.

The photometric quality is driven by the atmospheric extinction and emissionin atmospheric windows related to some atmospheric minor components, espe­cially H20 vapor, and requires long periods of stability of these properties. Theperformance in some infrared windows (4.5-5 J.lm, 20-28 J.lm, 34 J.lm with increas­ing dependence) is strongly related to the precipitable water vapor content whileimprovement is still important though not as drastic, in all other windows.

Seeing is obviously an important criterion at wavelengths at which backgroundthermal emission dominates i.e. above roughly 2.5 J.lm and below 10 J.lm where 8mpupils will often be diffraction-limited. In these conditions imaging S/N ratios,for instance, will increase as (seeing)-2 and the integration time as (seeing(Only observations (whatever the mode) of extended sources without structurewould not benefit from good seeing quality.

Below 2.5 J.lm the sky background is dominated by night airglow and, in twilighttime (01' daytime) by scattering. At tropical latitudes, auroral emission beingnegligible, night airglow is caused by hydroxyl (OH- radical) fiuorescence. OHairglow is not known to show any significant geographical variation within thearea surveyed (see 5.5.'1), so its amount does not appeal' to be a criterion ofselection.

It is worth mentioning here that the quality of a good site can easily be ruinedby inadequate telescope properties that would yield an unacceptable emissivity.Conversely a properly maintained telescope on a very dry site will give access toparts of the infrared range, such as Paschen alpha 01' 34 J.lm regions, quasi-opaqueat pOOl' sites and potentially very important scientifically.

6.2.3 Speckle lifetime

Speckle life time, known as the atmospheric coherence time to in Kolmogorov'stheory of atmospheric turbulence, has not played any important role in previoussite selections. The need for short exposures i.e. of exposure times upper lim­ited by to, basis of any interferometric work, now calls naturally for a change.In addition the systematic use of adaptive optics for non-interferometric workwill extend its infiuence basically to all operational modes. Therefore it shoulddefinitely be taken as a major selection criterion.

The actual reason for this emphasis is twofold: i) the speckle life time entersthe theoretical interferometric 01' adaptive optics S/N ratio expressions with ex­ponents higher than 0 and often than unity, the actual exponent depending on

CHAPTER 6. FIGURES OF MERIT 148

the type of noise limitation, the interferometric approach and the type of signalanalysis; ii) experience with present instruments on large telescopes demonstratesthat it has large variations over one or even two orders of magnitude, i.e. its vari­ations are likely to be even more troublesome for the operation than those ofthe coherence length (the so-called Fried's parameter ro). The latter fact can beexplained by thinking that while ro is a result of an integration over the lineof sight of local turbulence contributions, to is more dependent on the strongerair layers velocity gradient which, in turn, might be directly connected to thedichotomie situation of presence or absence of a jet-stream above the site.

Unfortunately this parameter is difficult to measure. It has not been the ob­ject of a systematic study with existing interferometric instruments, leaving theknowledge of it to a quite qualitative level. For instance, though it is observed todecrease (as expected) as a function of increasing spatial frequencies, i.e. makingits importance higher for long interferometric baselines, very scarce data of thissort have been coHeeted.

Speckle lifetime was measured directly during the LASSCA experiment (LaSilla Seeing Campaign) by MuHer ct al. [1] and indirect1y by Vernin et al. [2],using a Scidar and wind profiles from radiosonde data. A very good agreementwas obtained between the two methods using the following relation:

(6.1)

where ro is the seeing Fried parameter and ~V is the characteristic velocity ofthe turbulent layers weighted by the amount of thermal turbulence they carry.

This allows us to use such a model in the absence of direet measurements onboth sites. In a first approximation, as shown in section 4.6, the wind/turbulencedistribution may be considered as similar on both sites. The specklc lifetime maythen be included in the seeing exponent when relevant.

Bibliography:[1] M. MuHer, G. Baier, S. Helm, G. Weigeltj Optical Parameters of theAtmospherej Proc. NOAO-ESO conf. on: High-resolution imaging byInterferometry, Garehing, 15-18 March 88.[2] J. Vernin, G. Weigelt, J.L. Caccia, M. Müllerj Speckle life-time andisoplanicity determination from turbulence and wind profiles, to appear inAstron. Astroph.

CHAPTER 6. FIGURES OF MERIT

6.3 Determination of power laws

149

(6.2)

6.3.1 Direct imaging

-Visible wavelengthsWe assume that the V.L.T is mainly used for the detection of star-like objeetsagainst the sky background and we consider long exposure times compared toseeing fluctuations.

The limiting magnitude one can reach is affected by various parameters ofdifferent nature (atmosphere, instrument, detectors, observing conditions, etc... )as analysed by Baum (1962) in Astronomical techniques (Ed. W.A. Hiltner).As we want to compare sites, we do not consider factors related to telescopeor instrument and we assume an adequate sampling of the seeing disk on thedetector and unsaturated observations.

The remaining faetors are:

• effective integration time ~t ( photometrie nights)

• angular diameter of a star image 0 (seeing)

• sky background emission charaeterized by the sky emissivity €

For a star like objeet imbedded in a strong sky background, the SNR will be:

SNR <X n* {iioy-;,where n* is the stellar photon flux.

The most important for point like objeets is the seeing with apower factor oftwo because it affects only the signal emitted by astronomical objects, leavingunmodified the noise of the sky brightness considered as a Hat field.

On the contrary, the brightness of the sky only appears with apower factor ofone.

For extended objects considered as a Hat uniform source, the image qualitydoes not improve the final limiting magnitude, so the power factor is O.

-Infra-red wavelengthsThe parameters to be retained in the infrared domain are those given aboveplus the transmission T of the atmosphere which relates approximately, in thethermal bands (M to Q essentially), to the sky emissivity T = 1 - €.

As only low speetral resolutions are considered here since high resolutions aredealt with in seetion 6.3.2, the background photon flux is high, and one can thusconsidel' that the dominant noise will always be the sky background noise ab.While its theoretical expression:

(6.3)

CHAPTER 6. FIGURES OF MERIT 150

(6.5)

assumes photon noise statistics, the time and spatial variations of the back­ground aetually yield higher levels of noise. An approximation can therefore bederived by assuming a linear dependence of the sky background with emissivity

O'b cx: ELlto.5 0, (6.4)

where 0 stands for the seeing value to account for the adapted sampling.Then the SNR can be expressed:

SNR cx: Tn*Llt cx: T LltO.5

O'b EO.5tol 0

and the integration time scales as the power 2 of seeing (except in the Q bandwhere the telescope is diffraction limited), the power 1 to 2 of LR. emissivity,and the power 2 of transmission. To take into account the fact that the skybackground noise dominates the instrumental one in bands M and Q, the power2 of the emissivity is retained for them, while conversely we will keep apower 1elsewhere as summarized in Table 6.1.

Source Extended Point likeSpectral Band V,K,N V,K,N M Q

Seeing 0 2 2 0Speckle lifetime 0 0 0 0Sky emissivity 1 1 (1)2 (1)2Transmission 1,2,2 1,2,2 2 2

Spectroscopic nights no no no no

Table 6.1: Power relation between site parameters and dircet imaging

6.3.2 Spectroscopy

Let us first recall that we are asking by which power the various parameters enterinto the observing time nceded to reach a fixed signal to noise ratio (S/N). Asthe ratio between the total allocation time and the allocated hours will dependlinearly on the fraction of usable hours, the linear dependence on the fraction of(at least) spectroscopic hours is obvious.

Concerning the dependence on the other parameters, it is useful to considerthe following special cases:

1. If the sky background is negligible, the S/N usually depends only on theobjeet photon noise or on a fixed deteetor noise. In this case, all exponentsare zero.

2. If the sky background dominates, we have

SIN '" ßt... = n.J ßtJLltnsky nsky(6.6)

CHAPTER 6. FIGURES OF MERIT 151

where tlt, n*, and nsky are, respectively, the exposure time, the stellarphoton flux, and the sky photon flux for each spectral element. Hence, inthis case tlt will also depend linearly on the sky emissivity nsky' Concerningthe seeing dependence we can distinguish three basic situations (alwaysassuming a conventional grating spectrograph):

(a) If we use a fixed slit width and oversample on the detector along thedirection perpendicular to dispersion (as is usually done in the visual)bettel' seeing results in a concentration of the object spectrum on fewerdetector pixels and thus in a decrease of the underlying sky contribu­tion nsky proportional to the decrease of the seeing diameter. Hence wehave a linear seeing dependence of tlt (if we undersample, the powerwill be zero).

(b) If we undersample perpendicular to dispersion (e. g. in the LR. wherewe have large pixels and few elements), we can still gain from goodseeing by adjusting the slit width to the seeing, thus again loweringthe sky contribution. We again get a linear dependence of tlt on theseemg.

(c) Combining (a) and (b) (where possible) gives a quadratic dependenceon the seeing.

In actual astronomical observations, we normally encounter situations somewherein between the above sp.ecial cases. For high-resolution work (limited to relativelybright objects) in the V- and N-bands the sky contribution is usually negligible.Hence in this case we normally have situation 1) with no seeing dependence(exponent 0). However, at some wavelengths and under certain atmospheric con­ditions the sky contribution may become significant even for high resolutionspectroscopy. Hence the exponent can range between 0 and 2.

For low resolution spectroscopy in the V and K-bands, the sky is rarely negli­gible. Hence the cases 2(a) to 2(c) will apply (exponent 1-2) .

.In the thermal infrared bands the sky always dominates and detectors usuallyundersample. Hence we usually have the situation 2(b) (i.e. linear dependence).

Power relations are summarized in Table 6.2.

CHAPTER 6. FIGURES OF MERIT

Mode SpectroscopySource Extended Point like

Lambda V,K,N V K NResolution L II L II L II L

Seeing 0 (0)2 (1)2 (1)2 1 1 1Speckle L. T. 0 0 0 0 0 0 0

Skyemis. 1 0 1 (0)1 1 1 1Transmission 0 0 0 0 0 0 0Spect. nights y y y y y y y

Table 6.2: Power relation between site parameters and spectroscopy

152

6.3.3 Interferometry

Interferometric imaging with the VLT takes a number of different forms. Atthe individual telescopes it will use speckle imaging techniques at visible wave­lengths, but use image restoration by the VLT adaptive optics in the infrared.The figure-of-merit in the V-band will therefore be determined by different cri­teria than in the infrared because different techniques are used. At the combinedcoherent focus again different imaging methods will be used, and in an instru­mental configuration where almost always the instrument thermal emissivity willdominate over the sky emissivity.

Because of these and other factors the derivation of afigure-of-merit for in­terferometric imaging becomes complicated. We will below attempt to derivea single average figure-of-merit for interferometric imaging for each wavelengthregime, but indicate how these will differ for a specific application.

For the three wavelength bands (V, K, N) the sky emissivity f is relativelysmall (<< 1.0). For the latter (Q) it was assumed to be near 50 %.

Speckle lifetime, 01' atmospheric coherence time ta, also enters into the figure­of-merit estimates. It is also not measured. It will be taken as equal to ra/Vwind,where ra is the seeing (Fried's parameter) and Vwind the typical wind velocity forthe seeing layer.

-Visible Wavelengths (V band)Both the individual telescopes and the VLT Interferometer will work in themultis~cklemode. In that case for faint sources the SNR will be proportionalto py'M, where p equals the probability of having a photon in a speckle and Mequals the number of observations.

The probability pis proportional to r5.ta.T.BW, where Dis the telescope di­ameter, ra is theiseeing parameter, ta the speckle lifetime 01' the coherence time,and BW the spedtral bandwidth. Since ta is approximately equal to ra/Vwind, andsince BW for the largest spectral bandwidths of observations is proportional to~

rJ, p can increase very rapidly with seeing (proportional with rg·83 ). M decreases

CHAPTER 6. FIGURES OF MERIT

proportionally with increasing to. This then gives:

SNR cx: r 2.83 tO.5T NO. 5

° ° sp

153

(6.7)

since spectroscopic nights (Nsp ) are acceptable for interferometric imaging.On the basis of these considerations we therefore propose the following figure­

of-merit factors (rounded off to fuH numbers):

-seemg-speckle lifetime-sky emissivity-transmission-spectr. nights

61o2 (independent on emissivity)1

-Near-Infrared Wavelengths (K band)Now the functioning of the VLT adaptive optics becomes the critical parameterboth for single telescope and array interferometric imaging. Its limiting magni­tude is proportional to:

r5toT. (6.8)

IR emissivity is of little concern for adaptive optics per se, since detector read­out noise and object photon noise dominate. It is, however, of concern for theactual observations with use of the adaptive optics. These will have a limitingsensitivity which is determined (for a long enough integration to dominate theread-out noise) by the noise in the emissivity. We will assume here (and in thefollowing) that the sky emissivity noise is directly proportional to the emissivitysince inherent sky spatial and temporal variations tend to be higher than thepure photon noise. This results in sensitivities proportional to €-2 when the skyemissivities or its noise dominate. That will be (optimistically) assumed to bethe case for the single aperture mode; it will definitely not quite be the case forthe array mode where the instrument emissivity contributes significantly so thatsky variations become less of a factor.

Adaptive optics will not work at this wavelcngth at all sky positions, so thatthese results have to be combined with those discussed under the multispecklemode discussed above under the V-band.

On the basis of these considerations we therefore propose the following figure­of-merit factors (rounded off to fuH numbers):

-seemg-speckle lifetime-sky emissivity-transmission-spectr. nights

4222 (virtuaHy independent on emissivity)1

CHAPTER 6. FIGURES OF MERIT 1-54

-Mid-Infrared Wavelengths (N band)In the N (and Q) band the VLT telescopes are assumed to be always diffractionlimited either because the seeing is excellent, or because the adaptive opticssystem is capable of full sky coverage under any seeing condition. Interferometricimaging with single telescopes becomes therefore direct imaging and is coveredthere.

For interferometric imaging using the array, the SNR for a single exposure(which determines the limiting magnitude for interferometric observations) isdetermined by the rate of change of the fringe position (ta) and by the increaseof background noise which is now dominated by the instrument background emis­sivity photon noise (proportional to tg·5

):

SNR cx: tg·5T (6.9)

Sky emissivity noise contributcs probably less than instrument emissivity photonnOlse.

On the basis of these considerations, and on the basis that for a given observingtime, the number of exposures is proportional to tal, we therefore propose thefollowing figure-of-merit fadors:

-seemg-speckle lifetime-sky emissivity-transmission-speetr. nights

ooo21

-Far Infrared Wavelengths (Q band)Exactly like the N band case above, except that the noise in the thermal emissiv­ity of the sky becomes a major factor in determining the SNR. Ignoring the noisein the instrumental background the SNR for a single observation now becomesproportional to:

SNR cx: ta T (;-1 (6.10)

On the basis of these considerations, and on the basis that for a given observingtime, the number of exposures is proportional to tal, we therefore propose thefollowing figure-of-merit fadors:

-seemg-speckle lifetime

I

-sky emissivity!-transmission-speetr. nights

o1221

CHAPTER 6. FIGURES OF MERIT 155

-SummaryTable 6.3 summarizes the figure-of-merit numbers for VLT interferometrie imag­mg.

Spectral Band V K N QSeeing 6 4 0 0

Speckle lifetime 1 2 0 1Sky emissivity 0 2 0 2Transmission 2 2 2 2

Spectroscopic nights y y y y

Table 6.3: Power relation between site parameters and interferometry

6.4 Results

Sinee only some of the parameters used in the previous ehapters have been mon­itored, a simplified power relation is used for eaeh observing mode aeeording tothe following rules:-The speekle lifetime is included in the seeing power.-The sky emissivity in the Q band (measurement band of the KPNO water vapourmonitor) is directly available. In the M band it may be extracted by reproeessingthe water vapour data using an atmospherie model to eonvert emissivity at 20pminto emissivity at 10pm. It is not taken into aeeount in V, K, and N.-When photometrie and speetroseopie nights are aeeepted (Y), thc statisties ofspectroseopie nights are used with apower 1. When speetroseopie nights arenot aeeeptable (no), the pereentage of photometrie nights only is used with apower 1. This eould lead to underestimating the figures of merit (see note underTable 6.4).

The final relative merit of Paranal versus Vizeaehas is given in table 6.4, asdefined in seetion 6.1 and justified in seetion 6.3. The values given in Table 7.1were used for the eomputation of the eoeffieients. In addition, provisional num­bers were taken for emissivity and transmission at 20pm: the median emissivity,dedueed from water vapour eontent using information kindly made available byA. Morwood (LOWTRAN model), is 0.52 at Paranal versus 0.62 at La Silla.The transmission is taken as 1-emissivity as a provisional approximation (0.48at Paranal versus 0.38 at La Silla).

A few remarks have to be made eoneerning table 6.4:

1. For the Q band (sensitive to water vapour), the values given are to beeonsidered as eonservative as well as approximate, due to the uneertaintiesin the eonversions between emissivity, transmission, and water vapour;

2. in some eases, the values represent, in fact, lower limits: for instanee theamount of water vapour ean influenee the quality of speetroseopie studies,

CHAPTER 6. FIGURES OF MERIT 156

Mode Direct imaging Speetroscopy InterferometrySource Extended Point like Extended Point like Point like

Lambda V,K,N V,K,N M Q V,K,N V,K N V K N QResolution L L L L L n,L n,L L L L L

Seeing 0 2 2 0 0 2 1 7 6 1 2Skyemis. * * 2 2 * * * * * * 2Sky trans. * * 2 2 * * * * * * 2

Phot. hours 1 1 1 1 / / / / / / /Spect. hours / / / / 1 1 1 1 1 1 1

Ment 1.4 1.9 3.1 I 1.1 I 1.5 1.3 , 2.9 2.5 1.3 3.3 ITable 6.4: Power relations and relative merit of Paranal versus La Silla as a funetionof observing mode, quantities with an asterisk have not been measured.

an example being in high speetral resolution in the visible;

3. it should also be stressed that the use of the V.L.T. for speetrophotome­try (in partieular of faint objects) or for very high resolution speetroseopywill undoubtedly be large, and, therefore, that the requirement for "photo­metrie" nights for "spectroseopy" will be important. Here again the givenfigures of merit ean be eonsidered as lower limits.

As a eonclusion eoneerning the figures of merit given in t,able 6.4, one may statethat, sinee all values are larger than unity, it is for all the eases eonsidered herethat the Paranal area appears to be superior to the one around La Silla.

6.5 Figures of excellence

Another way of eomparing sites would be to eompute the fraction of the timeduring whieh all parameters were simultaneously inside a range defined as opti­mum for astronomy in general. A tentative seleetion of eriteria is the following:Photometrie night, wind> 2m/s and < 10m/s, seeing< .5" (during 1 hour ),humidity< 90 % and preeipitable water vapour less than 1 millimeter.

Table 6.5 undoubtedly shows the superiority of the Paranal area over that ofLa Silla under the very stringent eonditions that have led to the given values:this appears clearly in terms of number of photometrie nights, very low precip­itable water vapour, number of nights with aceeptable humidity and frequencyof exeellent seeing.

It is expeeted that the V.L.T.'s maximum efficieney will, for engineering rea­sons, take plaee for winds having a speed less than 10m/s, which is in favour ofLa Silla (6% mote time). Yet in open struetures, some wind (2m/s) is neeessaryto prevent the heat to aeeumulate around the optical path. Combining the tworequirements, Paranal eomes out better by 6%.

CHAPTER 6. FIGURES OF MERIT 157

Parameter Paranal area La Silla area Ratio% nighttime % nighttime ParanalJLa Silla

Photometrie sky 81 58 1.40Water Vap. ::; 1mmH2O 8.2 3.7 2.2

2 ::; Wind ~ 10 m/s 68 64 1.06Humidity ~ 90 % 100.0 96.0 1.04Seeing ~ 0.5 " (*) 16.2 6.8 2.4

Tahle 6.5: Frequency of occurrence of excellence of parameters at hoth sites duringcommon periods of time, hut without requirement of coincidence, (*)=update Sep89­Aug90.

On the other hand, high windspeeds (above 15m/s) which can degrade orprohibit observations, are more frequent at Paranal (7% of nighttime) than atVizeaehas (4%). Though seeing was not monitored under high wind, the dataobtained at Paranal between 10 and 16m/s do not show a correlation betweenwind velocity at ground level and atmospheric seeing.

In any case extreme care will have to be taken in ehoosing the VLT configura­tion so as not to perturb any of the four units by one (or more) other teleseopes.Appendix A is illustrative on this matter since it considers how the seeing can beaffected by structures located upwind: appendix A indeed deals with monitoringthe seeing in the wake 0/ the NTT building.

If, however, one were now to consider opportunities of observing under allexcellent conditions together (assuming they are totally uncorrelated), then hewould be witnessing a gain of a factor of up to around 8 between the Paranaland the La Silla areas.

As illustration of the quality of the sites, Figures 6.1 and 6.2 respectivelyrepresent what is so far the best complete night of observation with the bestseeing achieved on Paranal and on Vizcachasj houdy averages are indicated bythe horizontal bars.

CHAPTER 6. FIGURES OF MERIT 158

1 Table:

.9

.8

.7

~.61II

~~.5

E..c:~ .4

.3

.2

.1

CERRO PARANAL

September 20, 1990

' .... ". ..........~~. ,': ~..~.: .",', ""~".. .~.'" ',. .. .. "... .. "~ " .':: ~~'. ......-r:.~:..~. '.,.: ...........:....~ :-~ .. " .. . .·.·~r-"" •.

".~~ -.- r- \.. . " .

Night Average=0.32 aresee

0'---'--.........--J.---''---'----L..--L..---l_-'---..L.--L--l._L-...L---L.--L..--l_..L-..L..-J

o 2 3 4 5 6 7 8 9 10time (Hour-UT)

Figure 6.1: Best night at Paranal: thc dots are individual measurcments by thc DIMM;horizontal bars correspond to one hour linear binning. The sceing is defined as theFWHM at zenith, at wavelength O.5I1m, and is mcasured 5 metrcs above ground onthe summit.

1 Table:

.9

.8

.7'.

CERRO VIZCACHAS

Oelober 15, 1989

E'i.4-

0' .~:Q).6 .. .. ......" ..~ ...:..::-.. .. "" " .... '.".. .. ", '.'. .... " - ".', .~\.~ ...~.5.. ".~ .:...::-.... " .. ~'''--'

~ ,~ .~ " .. \ ~ .. " ".:..~. f'.. ., " ':- ----=......... 't ..''\10 :.. .. _." .. .. .. .. liII ........ ..... .. "

.3

.2

.1 Night. Average=0.49 aresee

o '---'--..................---''---'---'---'----'_-'---'---'----'_''---'''''--'---'-._'---'--.........-Jo 2 3 4 5 6 7 8 9 10

time (Time-UT)

Figure 6.2: Bes~ night at Vizcachas: the dots are individual measurements by theDIMM; horizontal bars correspond to one hour linear binning. The seeing is defined asthe FWHM at zenith, at wavelength Q.5I1m, and is measurcd 5 metres above groundon thc summit.

Chapter 7

RECOMMENDATIONS

1 The VLT Site Seleetion Working Group (SSWG) has considered the scientifi.ccharaeteristics of chilean sites in the Paranal area and in the La Silla area.

The extensive set of relevant astronomical and meteorological data (of whichsummaries appear in this draft report) has been examined and analyzed.

It is clear from the analysis presented in Chapters 5 and 6, that the overallremarkable quality of the Paranal area is superior to that of the La Silla area, asillustrated in the table 7.1 below.

Phot. nights Spectr. nights Seeing Water vapour(%) (%) arcsec mmH20

La Silla area 58 82 0.76* 3.9Paranal area 81 90 0.66* 2.3Ratio Earanl!:! 1.42 1.11 1.15(i) 1.70(i)~~

Table 7.1: Median values of measured parameters at both sites during common periodof time, (i)::inverse ratio, (*)=update Sep89-Aug90.

On the basis of scientific considerations, therefore, the SSWGunanimously recommends that the Paranal area be chosen for the

location of ESO's Very Large Telescope

Comparing Cerros Paranal and Armazoni, while it is recognized that Paranalhas been investigated to a much larger extent than Armazoni, from the dataavailable it is hard to distinguish between the two summits as the site for theV.L.T. In any case, given the unique qualities of the two summits, the SSWGrecommends that efforts be taken to retain both sites for the purpose of theV.L.T. and further developments.

1unanimously formulated by the non ESO members of the panel

159

Chapter 8

ACKNOWLEDGMENTS

This report represents a summary of data collected during more than six years ofefforts, often in very difficult conditions, by a number of people in Chile, whomthe members of the Site Seleetion Working Group would like to thank, in thename of European Astronomy.

Among the many contributors to the success of this study, a partieular mentionis addressed to:

• The members of the ESO-VLT site group, head: H. E. SchusterRoberto Castillo, Bruno Lopez, Julio Navarrete, Ariel Sanchez and formerlyFranl,tois Rigaut, Bertrand Koehler.

• The Paranal teamFrancisco Gomez senior, Francisco Gomez junior, Italo Gomez, Daniel Maza,Alfonso Vargas, and formerly WeIlington Vega.

• The technical maintenance teamManfred Morhinweg and Gero Timmermann from the TRS group (head:D. Hofstadt) at La Silla. .

• The members of the Maintenance and Construetion group (head T. Hoog)at La Silla

• The drivers

• And last but not least, the cooks !

160

Appendix A

MONITORING THE SEEINGIN THE WAKE OF THE NTTBUILDINGS

Author: Marc Sarazin, July 19, 1990

The NTT area was used as a model to estimate the level of microthcrmalfiuetuations induced by the aerodynamical wake of buildings. This paper showsthat the wake turbulence is easily distinguishable from the ground turbulence

because of a different dependence on wind speed.

A.l Introduction

The VLT current lay-out is such that a unH Sm telescope will be under the windof at least one of its neighbors during a non negligible fraction of the observingtime. Moreover, the interferometric auxiliary telescopes will be under the wind ofthc Sm units and of thc interferometric and combined focus laboratories duringmost of the observing time. A number of studies have been undertaken since

161

APPENDIX A. SEEING IN THE WAKE OF THE NTT BUILDINGS 162

1989 to determine whether such a configuration will affect image quality and, ifso, by how much.

Already in 1986, during the LASSCA experiment, a dramatic increase of theseeing had been monitored in the wake of the 1m Infrared telescope at La Sillal.This can be understood since concrete buildings like the enelosure of the 1mtelescope have a high thermal constant which prevents them to reach thermalequilibrium with the ambient air.

After completion of the NTT, a first campaign of measurements using onedifferential image motion monitor (DIMM) revealed that the seeing in the wakeof the NTT building area was most of the time worse than the seeing measured atthe NTT itself, but also that this amount of additional seeing seemed to decreasewith increasing wind velocity2. The NTT enclosure, covered with aluminum, hasa low thermal constant and should disturb the incoming flow only mechanicaIly,without introducing heat sources. On the other hand, the small services buildingis not thermally insulated, neither are the telescope access roads.

A theoretical analysis using a simplified model was proposed by L. Zago andapplied to the VLT unit 8m telescopes3 . A more detailed theoretical study isunder way, using a 3 dimensional numerical model at Risoe Laboratories (Den­mark).

The weak point of the first NTT experimental campaign is the unability toseparate wake effects from ground effeets. The DIMM, sitting only at two metersabove asphalt, was also measuring some local ground turbulence which was notcreated by buildings and which would not affect a large tclescope whose primarymirror is at least 10m above ground level.

The second NTT campaign described below, was conducted during the wholemonth of January 1990, in an attcmpt to overcome this ambiguity without re­quiring specific infrastructure work.

A.2 Instrumentation

As in the first NTT campaign, a seeing monitor (DIMM2) is locatcd 40m Southof the building, 2m above asphalt. Then, a second seeing monitor (DIMM3) isinstalled, also at 2m above ground, elose to the Schmidt telescope at a place freefrom wake effeets for the most frequent wind direetions (North-South). Fig. A.lshows the location of the two seeing monitors. Each DIMM measures seeing andlow frequency (50Hz) scintillation approximately every 100 seconds.

Wind speed and direction, temperature, pressure and relative humidity aremonitored at the Vaisala meteo mast (20 minutes average), 1km to the West.

i

1 Lasscaj La Silla ~eeing Campaign, Data analysis Part I, Seeing; December 1987, VLT Report nOSSpage:28.

2Int. Memo. WP-S046-30, Nov. 3, 1989 by M. Sarazin, R. Castillo, B. Gilli.3L. Zago, Local seeing in the VLT telescope area; Technical note TE-BOE-l, 30 March 1990.

APPENDIX A. SEEING IN THE WAKE OF THE NTT BUILDINGS 163

Wind speed and direction are also monitored at the location of DIMM3 (lminuteaverage).

A.3 Methodology

During the month of January 1990, more than 1500 simultaneous individualseeing sampIes were recorded on each site. The corresponding 5 minute bins aregiven on Fig. A.2 showing that the seeing behind the NTT was most of the timeworse than at the reference site.

The seeing records are split into two classes according to wind directions suchthat DIMM2 sits either inside or outside the wake of the NTT enclosure andservice building.

In each case, the differential seeing between the tested site and the reference site(DIMM2 minus DIMM3) is analyzed as a funetion of wind velocity. Yet, becauseseeing is not linearly additive, we use the integral of thermal disturbances (Cl)as a parameter, noting that:

(A.1)

(A.2)

We then define a differential thermal turbulence between the locations of thetwo DIMMs:

ß(J Cn cx [FWHMl)JMM3 - FWHMbIMM2]

Since we search for an effect of a building on an already perturbed atmospherc,we get rid of upwind seeing fluctuations by rather studying the relative differ­ential thermal turbulence:

5 5

ß(J Cl) FWHMhIMM3 - FWHMhIMM2 (A.3)7J = JCl(h)dh = FWHMi

DIMM2

We then obtain a parameter 7J which includes only ground effects when DIMM2sits outside the wake, and ground+wake effeets in the other case. One can thenretrieve the wake effect by subtracting the ground effeet with the following as~

sumptions:

1. The ground turbulence at the tested site (DIMM2) is the same for any winddireetion.

2. The ground turbulence at the reference site (DIMM3) is negligible for allwind directions.

3. Both ground turbulence and wake turbulence depend on the wind velocity.

APPENDIX A. SEEING IN THE WAKE OF THE NTT BUILDINGS 164

Figure A.1: Location of the seeing monitors: DIMM2 is south of the NTT building andDIMM3 is east of the Schmidt telescope.

APPENDIX A. SEEING IN TIIE WAKE OF TIIE NTT BUILDINGS 165

f- I

1. 5 '-

-

-

----

.5 '-

- ro

I

I200

I

I400

I

I

600

-

---

------

-

SEQUENCE

Figure A.2: The seeing at the reference site (DIMM3, fullline) is compared to thc mea­surements behind the NTT buildings (DIMM2, doted Une) as a function of recordingsequence.

Assumption 1 is justified by the presence of asphalted roads on all sides aroundDIMM2. Because assumption 2 is not fulfilled all the time, we choose to workonly with positive values of 'fJ. Assumption 3 is obvious on Fig. A.3 and agreeswith the results of the 1989 campaign.

For each dass of wind direction, a best fit is determined between relative differ­ential thermal turbulence ('fJ) and wind speed. The best fit ('fJw) for the relativecontribution of the wake is deduced from the difference of the fits correspondingto measurements taken inside and outside the wake.

The analytical relation obtained is then used with Eq. A.3 to determine thedifferential seeing due to the wake only:

A.4 Results

:1

ß w(FWHAf) = FWHMDlMM3 'fJ"J (AA)

A.4.1 Selection of the wind direction

As shown on Fig. AA, the DIMM2 monitor will see the wake of the telescopeenclosure and of the service building for almost any wind coming from the twonorthern quadrants. On the other hand, the southern side is free from any con­struetion.

APPENDIX A. SEEING IN THE WAKE OF THE NTT BUILDINGS 166

4

~

Cl1Il

rel-

~

~2Cll-

. ..' .: . '.

'..

N~

Ul­'C

o .' ••':~.::' '0. '': .....

0246851 5 (rrt»

Figure A.3: Relative differential thermal turbulence as a function of wind speed for allwind directions.

Seeing measurements with wind coming from both directions were necessaryfor the purpose of our comparative analysis. Fortunately, as can be seen thenighttime windrose recorded at the Vaisala tower, shown on Fig'. A.5, the windblcw from the South during one third of the time.

AA.2 Determination of the relative thermal turbulence in the wake

After separation of the records into two classes, it appears that the North windrecords are consistently higher than South wind records.

The cases of negative differential turbulence represent 18% of the total numberof sampies. Half of them correspond to cases of equal seeing on both locations (theaccuracy on the estimate of the differential turbulence is evaluated to ±15%).The remaining half is due to local effects at DIMM3 level. The latter cases arediscarded in what follows.

Fig. A.6 shows the results of fitting polynomials to the sampies collected withNorth or South wind, and of their difference corresponding to the wake effect.Some comments are necessary:

• Both South and North fittings reach the same value near zero wind veloc­ity. The turbulence is then purely from ground, which confirms our firstassumption of section A.3.

• The South wind fitting is not accurate at high wind velocity due to thescarcity of sampies. As a consequence, the difference fit becomes negative

APPENDIX A. SEEING IN THE WAKE OF THE NTT BUILDINGS 167

Figure A.4: Top view of the NTT area with the service buildings and asphalted accessroads surrounding the seeing monitor (scale 1:1000).

APPENDIX A. SEEING IN TIIE WAKE OF TIIE NTT BUILDINGS 168

10

1j.

...5

......\ll

IIII-XIII

0

syls (m/~)

Figure A.5: Windrose at La Silla during the mcasuremcnts. South wind represents onethird of the sampIes.

above 5m/s . We assurne that for higher wind speed, the contribution ofthe wake becomes then negligible.

A.4.3 Wake equivalent seeillg

The resulting FWHM due to the wake is shown on Fig. A.7. It is computedas shown in Eq. AA, applying the polynomial fit to the North wind sampiescollected at the reference site.

APPENDIX A. SEEING IN TlIE WAKE OF THE NTT BUILDINGS 169

.1.5

----------

""

I

I

/

\\

\\

\\

\\

"\

"-~ .. ., "'" .....". .. .

/ ......_.-...------

......

.5

N...u~'0......4-.

oe...--:---'--'--'---::--'----L---I-~-L-~~-L-....I-~---l.-L--l-.....LJo 2 4 6 8

sls (m/s)

Figure A.6: The fitted polynomials expressing relative differential turbulence as a func­tion of wind speed for North wind records (fuU line), and for South wind records(dashed line). The difference fit (dash-dotted line) corresponds to the contribution ofthe wake.

.2

. .'

o 2 4 6 8sls (m/s)

Figure A.7: The contribution of the wake to image size versus wind velocity: the wakepolynomial fit is applied to the North wind seeing sampies recorded at the referencesite.

APPENDIX A. SEEING IN THE WAKE OF THE NTT BUILDINGS 170

A.5 Conclusion

The results of a month of measurements are summarized in Table A.1. It showsan increase of 40% of the seeing value when observing downwind of buildings afew meters above ground level. It is estimated that one half of that amount isdue to the wake of the buildings. The rest has to be attributed to local groundturbulence which may be suppressed by a proper treatment of road surf",ccs.

Parameter Seeing (arcsec)5% 50% 95%

Site (DIMM3) 0.49 0.68 0.90Site+wake (DIMM3+fit) 0.60 0.85 1.17

Site+wake+ground (DIMM2) 0.68 1.03 1.42

Table A.l: Seeing statistics for 700 North wind sampies.

Those measurements are quite representative of the conditions that could bemet by the VLT auxiliary telescopes unless great care is taken to make the VLTbuildings and laboratories "transparent" both thermally and aerodynamically.The relation between wake turbulent strength, wind velocity and distance to thebuilding remains to be found.

Since the 8m VLT unit telescopes are much higher above ground, their levelof interaction depends on the altitude distribution of the turbulence inside thewake. Nevertheless, the seeing increase on a 8m unit sitting in the near-wake ofanother, could be as large as stated in this report if, as assumed in L. Zago'spaper, the turbulence is concentrated in the shear layer rather than uniformlyvertically distributed. '

To conclude on the VLT 8m units, the knowledge of the altitude profile ofthe turbulence is thus necessary. This can be obtained experimentally using mi­crothermal C; sensors on a mast a few tens of meters away from the enclosure.

Acknowledgments:This study was made possible by the contribution of the members of theVLT-site group, ESO-Chile.

Appendix B

SEISMIC HAZARD IN THEVIZCACHAS AND PARANALAREAS

Author: Fabien Bourlon, September 17, 1990

B.I Seismicity in Chile

Chilean seismic activity is mainly linked to a subduction zone. Fig. B.I showshow the South American plate moving westwards covers the Nasca plateadvancing eastwards. The North-South trench located in the Pacific, parallel tothe coast, marks the front of this overlap. This relative movement is estimatedto be about 10 cm/year. The seisnllc source is mainly located on the slope ofthe downgoing slab, the Benioff zone, where the frietion between the two platesliberates energy. There is an intra~plate activity, but it is consideredunimportant regarding. the seismic hazard. It is linked to localized geologicalcomplexities. It can be noted that both the Vizcachas and Paranal sites arelocated near the coast line, at the border of the main seismic source, and Westof the intraplate source (the Andes mountains).In the following paragraphs data from the International SeismologicalCenter [I.S.C.86], from Barrientos's seismic study of Chile [Barrientos 80] andfrom R. Araya's seismic hazard study [Araya 90] are considered. Two scales ofanalysis have to be adopted in this seisnllc study leading to regionalconsiderations on the one hand, and to local considerations on the other hand.It should be noted that the magnitude on the Richter scale gives the quantityof energy freed from an earthquake. But in order to estimate the effect on aparticular site, it is needed to consider the distance from the site to the event'sepicenter: this effect is the intensity or the peak horizontal acceleration.

171

APPENDIX B. SEISMIC IIAZARD

wTl?ENCH

PACIFIC OCEAN

NAZCA PLATE

~ BEN/OFF SEISHI C SOURCE

~ INTRAPLATE EARTHQ.UAKE SOUR CE

COAST UNE ANOES HOUNTAINS

172

E

Figure B.l: Schematic profile of the subduction zone [Araya 90].

B.2 Seismicity in the Vizcachas and Paranal areas

B.2.1 International Seismology Center Data Base

• Regional analysis: As far as the regional aspect is considered the Paranalarea is less subjeet to earth tremors than the Vizcachas area. The datarecorded at the International Seismological Center: from January pt, 1964to February 30th , 1986 (period of time when seismic observations can beconsidered most reliable and complete) [I.S.C.86) shows that there occured9265 earthquakes in the Vizcachas region and only 5573 in the Paranalregion (an average of respectivly 1.1 and 0.7 events per day) .

• Local analysis: Precisely on the Paranal site, 2012 events are estimatedto have had notable effect (with a peak horizontal acceleration of at least0.0019) against only 1667 such events at Cerro Vizcachas. In other words,statistically, notable events happen every 4 days at Paranal and only every5 days at Vizcachas. In the past 22 years the highest peak horizontal ac­celerations estimated equal to 0.2089 ± 50% at Paranal (Sept 8, 1967) and0.4899 ± 50% at Vizcachas (Nov 15, 1967).

B.2.2 Regional (,analysis of seismicity in Chile

According to [Barrientos 80) the two regions where the sites are located haveslightly different features due to the local disposition of the tectonic plates.Fig. B.2 shows the seismic zones and the seismic parameters linked to each of

APPENDIX B. SEISMIC HAZARD 173

themj along is given the Maximum Magnitude Recorded (MM R) betweenparenthesesj Paranal is part of zone B (with M M R of 7.5) while Vizcachasbelongs to zone C (with M M R of 8.4).The. /requency 0/ occurrence 0/ seismic events in the two regions is given by theGutenberg-Richter law

IOglO(N) = a + b x M, (B.1)

where N is the annual number of earthquakes, M the magnitude considered, a

and b the regional parameters given by Barrientos on Fig. B.2. (Fig. B.3). Byintegrating these equations (Fig. B.3), we see that the annual frequency ofoccurrence of earthquakes of magnitude 5 to 7.5 is higher in the Vizcachasregion (there are annually 0040 such events in the Vizcachas region and only0.22 in the Paranal area). For earthquake magnitudes of 7.5 to 8.5 thisoccurence is slightly more important in the Paranal region than in theVizcachas one (respeetively 0.0034 and 0.0031 annual events).

B.2.3 Local Analysis of Seismic Hazard on Cerro Paranal and CerroVizcachas

In his seismic hazard analysis for the two sites, [Araya 90) processes recordedregional events and computes the accelerations which would have beenmeasured on Cerro Paranal and Cerro Vizcachas as shown in Fig BA.He evaluates that in aperiod of 25 years and with a 50% probability ofexceedance defining the Operating Basis Ea'rthquake (OBE), Paranal would besubjeet to one seismic event with a Peak Horizontal Acceleration (P HA) of0.16g ; the equivalent of an earthquake of magnitude M = 7.75 on the Richterscale with a distance to focus R = 100km. At Vizcachas the correspondingp HA would also have a value of 0.16g but would correspond to a magnitude M= 7.0 with a distance to focus R = 50km.Aperiod of 100 years and a 10% probability of exceedance correspond to theMaximum Likely Earthquake (MLE). The values of the P HAare then 0047g(with M = 8.25 and R = 1l0km) for Paranal and 0.50g (with AI = 8.5 and R= 150km) for Vizcachas.The differences of PHA from site to site being small in front of the standarddeviation of 0.794 given in [Araya 90), p15 on Peak Horizontal Acceleration, itcan be concluded that the seismic hazard at the two sites is similar.

APPENDIX B. SEISMIC HAZARD

82° 80° 78° 76° 7~0 72° 70° 6&° 66°

174

ZONE a bAe ~ .35 -.HC; 5.25 -.no ~." -."E '.51 -1.10FG 5.81 -'.16H 4." -\03

Antofagasta --------------~-JParanal

Vizcachas (La 5i11a)-------440(

5antiago --------J::="#:r-f--j

18°

20"

22°

2~0

26°

28·

30°

32"

36°

38·

~OO

~2°

~~.

~6'

Figure B.2: Seismic sources for chile [Barrientos 801·

APPENDIX B. SEISMIC HAZARD 175

o:J

2 0.001«

...\l)

.DE 0.01:JZ

-- Cerro Paranal

..." ... - - - Cerro Vizcachas

..............,

' .... ... ... ... ,' .. ... .. ....

10":1""""-----------------,

0.1'+-o

I/)IV.xo:J0­

.t:.......ow

0.00015.00 6.00 7.00 8.00

Magnitude

Figure D.3: Gutenberg-Richter Law for the Vizcachas and the Paranal seismic regions(modified from [Araya 90]).

250

= 50

= 100

••••• Cerro Vizcachns

- Cerro Paranal

~0IV>-.l-

.SIVUC0

"0 0.1Q)Q)u~"-o

E:.5o

.D

en.

0.010.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Peak Horizontal Acceleration (a)

Figure D.4: Seismic Hazard Curves for Cerro Vizcachas and Cerro Paranal (modifiedfrom [Araya 90]).

REFERENCES

B.3 Conclusion

176

B.3.1 Regional aspect

From the ISC data base and the Barrientos study it can be seen that theVizcachas region is subject to more earthquakes than the Paranal region. Alsothe maximum recorded earthquake given by Barrientos took place in theVizcachas region.

B.3.2 Local aspect

In the 22 year period going from 01/01/64 to 30/02/86, seismic events withnotable effect (PH A of O.OOlg) were more numerous on Cerro Paranal than onCerro Vizcachas. Still the most important event occurred on Cerro Vizcachas.Yet the seismic hazard as described by R. Araya seems equivalent on both sites.As far as constructions are concerned, stuctural criteria to be considered willnot significantly differ on both sites.In further studies, an investigation of microseismic activity should be madebecause events of lesser intensities than those considered in the SeismicH azard Analysis of [Araya 90] might be detrimental to astronomicalobservations (interferometric mode). In particular, after-shock re-alignmentprocedures should be examined in detail.

References

[I.S.C.86]

[Barrientos 80]

[Araya 90]

International Seismological Center, Seismicity 0/ Chile andReunion. Newbury, Berkshire, RG13 1LZ United Kingdom(30/02/1986). ESO purchase # 17895/4292Barrientos S. Regionalizacion Sismica de Chile Thesis deMagister en Ciencias con Mencion en Geofisica, Universidadde Chile (1980).Araya R. Seismic Hazard Analysis (1990). ESO contract#058/90