Sardinia Radio Telescope: General Description, TechnicalCommissioning and First Light

P. Bolli*, A. Orlati†, L. Stringhetti‡,¶, A. Orfei†, S. Righini†, R. Ambrosini†,M. Bartolini†, C. Bortolotti†, F. Bu®a§, M. Buttu§, A. Cattani†, N. D'Amico§,

G. Deiana§, A. Fara§, F. Fiocchi†, F. Gaudiomonte§, A. Maccaferri†, S. Mariotti†,P. Marongiu§, A. Melis§, C. Migoni§, M. Morsiani†, M. Nanni†, F. Nasyr§, A. Pellizzoni§,

T. Pisanu§, M. Poloni†, S. Poppi§, I. Porceddu§, I. Prandoni†, J. Roda†, M. Roma†,A. Scalambra†, G. Serra§, A. Trois§, G. Valente§, G. P. Vargiu§ and G. Zacchiroli†

*INAF – Osservatorio Astro¯sico di Arcetri, Florence, Italy†INAF – Istituto di Radio Astronomia di Bologna, Bologna, Italy

‡INAF – Istituto di Astro¯sica Spaziale e Fisica Cosmica di Milano, Milan, Italy§INAF – Osservatorio Astronomico di Cagliari, Cagliari, Italy

¶ luca@iast.milano.inaf.it

Received 2015 May 5; Revised 2015 September 15; Accepted 2015 November 30; Published 2016 January 18

In the period 2012 June–2013 October, the Sardinia Radio Telescope (SRT) went through the technicalcommissioning phase. The characterization involved three ¯rst-light receivers, ranging in frequency be-tween 300MHz and 26GHz, connected to a Total Power back-end. It also tested and employed thetelescope active surface installed in the main re°ector of the antenna. The instrument status and perfor-mance proved to be in good agreement with the expectations in terms of surface panels alignment (atpresent 300!mrms to be improved with microwave holography), gain (!0.6K/Jy in the given frequencyrange), pointing accuracy (5 arcsec at 22GHz) and overall single-dish operational capabilities. Unresolvedissues include the commissioning of the receiver centered at 350MHz, which was compromised by severalradio frequency interferences, and a lower-than-expected aperture e±ciency for the 22-GHz receiver whenpointing at low elevations. Nevertheless, the SRT, at present completing its Astronomical Validationphase, is positively approaching its opening to the scienti¯c community.

Keywords: Radio telescope, technical commissioning, radio astronomy.

1. Introduction

The Sardinia Radio Telescope (SRT) (Lat.39"29 034 00N – Long. 9"14 042 00E; 600m above the sealevel) is a new Italian facility for radio astronomywhose commissioning was completed at the end of2013. Its formal inauguration took place on 2013September 30. The antenna is a fully steerable,wheel-and-track dish, 64m in diameter, located35 km north of Cagliari, on the island of Sardinia. Itcompletes a set of three antennas devoted to radio

astronomical science in Italy (Fig. 1), all managedby the Italian Institute for Astrophysics (INAF).

The SRT is a general-purpose radio telescopeaimed at operatingwith high aperture e±ciency.Onceall the planned devices are installed, it will observe inthe frequency range from 300MHz to 100GHz andbeyond (from 1m to 3mm in wavelength). The an-tenna gain is expected to vary from 0.50 to 0.70K/Jyfor the frequency range 0.3–50GHz and to be around0.34K/Jy in the 3mm band (70–115GHz).

A key feature of the SRT is its active surface, intotal composed by 1116 electromechanical actua-tors, able to correct the deformations induced bygravity on the primary surface (or \mirror"). E®ortis underway to employ this facility to also correct

This is an Open Access article published by World Scienti¯cPublishing Company. It is distributed under the terms of theCreative Commons Attribution 4.0 (CC-BY) License. Furtherdistribution of this work is permitted, provided the originalwork is properly cited.

Journal of Astronomical Instrumentation, Vol. 4, Nos. 3 & 4 (2015) 1550008 (20 pages)#.c The Author(s)DOI: 10.1142/S2251171715500087

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for non-systematic errors, such as temperature/wind-related e®ects.

The SRT is capable of hosting many microwavereceivers, located in four di®erent antenna focalpositions — primary, secondary and two beam-waveguide foci — able to cover almost continuallyits frequency range. The SRT will operate in single-dish (continuum, full Stokes and spectroscopy),Very Long Baseline Interferometry (VLBI) andSpace Science (Ambrosini, 2011a) modes.

Thanks to its large aperture and versatility(multi-frequency agility and wide frequency cover-age), the SRT is expected to have a major impact ina wide range of scienti¯c areas for many years tocome. A full description of the potential SRT sci-ence applications is beyond the scopes of the presentpaper, as it is provided in a separate paper dedi-cated to the Astronomical Validation activities(Prandoni et al., in preparation). Here we illustratesome of the main areas where we think the SRT canplay a major role in the next future.

Operations in the framework of internationalVLBI and Pulsar Timing networks are of top pri-ority for the SRT. SRT is going to be one of the mostsensitive European VLBI Network (EVN) stations,together with E®elsberg and Jodrell Bank. Its largeaperture is also of extreme importance for SpaceVLBI observations with RadioAstron. Thanks to itsactive surface, the SRT will also represent a sensitiveelement of the mm-VLBI network operating at 7-and 3-mm bands, where a substantial improvementin collecting area and in the coverage of the skyFourier transform plane is of vital importance forincreasing the number of targets accessible to thearray and the quality of the images. Once the ¯ber

optic connection to the site is completed, the SRTwillalso participate in real-time VLBI observations (e-VLBI). The availability of three antennas (Fig. 1)will moreover allow the constitution of a small ItalianVLBI network, exploiting a software correlator(DiFX, already operating). The SRT will be also in-cluded in the geodetic VLBI network.

The SRT is one of the ¯ve telescopes of the Eu-ropean Pulsar Timing Array (EPTA), which, togetherwith the North American Nanohertz Observatory forGravitational Waves (NANOGrav), and the ParkesPulsar Timing Array (PPTA) share the goal to detectgravitational waves. Being the southernmost telescopeof the EPTA collaboration, the SRTwill allow a bettercoverage of pulsars with declinations below #20",hence a better overlap with the PPTA. Thanks to itsdual-band L/P receiver, SRT will be of great impor-tance in measuring accurate dispersion measure var-iations, crucial to obtain ultra-precision pulsar timing,and search for signatures of space-time perturbationsin the pulsar timing residuals. The SRT is also part ofthe Large European Array for Pulsars (LEAP), aproject which consists in using the EPTA telescopes intied-array mode, obtaining the equivalent of a fullysteerable 200-m dish.

The SRT is expected to have a major impactalso for single-dish observations. In particular, weaim at exploiting its capability to operate with highe±ciency at high radio frequency. Equipped withmulti-feed receivers the SRT can play a major rolein conducting wide-area surveys of the sky in a fre-quency range (20–90GHz), which is poorly explored,yet very interesting.

For instance the ¯rst-light K-band 7-beams re-ceiver will be exploited to obtain extensive mapping

Fig. 1. The SRT and the location of the three antenna sites.

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of the NH3 in Galactic star-forming regions, in closesynergy with existing IR/sub-mm continuum sur-veys of the Galactic Plane. The ammonia molecule,through its (1,1) and (2,2) transitions, is a goodtracer of dense cores of molecular gas and is con-sidered an excellent thermometer. Another inter-esting application of the K-band multi-feed receiveris a H2O line survey of nearby galaxies. This willincrease the number of detected water masers andderive distances, dynamical models and total massesof (luminousþ dark) matter of the galaxies in theLocal Group.

Wideband multi-feed receivers operating athigher frequency (40–90GHz) — currently underdevelopment — will allow us to get access to uniquemolecular line transitions in our own Galaxy, likefor example those associated with deuteratedmolecules (e.g. DCOþ ð1#0Þ, N2Dþ ð1#0Þ), cru-cial to constrain the kinematic and chemical prop-erties of pre-stellar cores, as well as to uncover thecool molecular content of the Universe in a crucialcosmic interval (redshift !0.3–2), through themapping of redshifted CO low-J transitions. Wide-band radio-continuum (and spectro-polarimetry)mapping of extended (low-surface brightness) Ga-lactic and extra-galactic sources (e.g. SNRs, radiogalaxies, nearby spirals), on the other hand, willpermit resolved studies aimed at a better under-standing of the physics of accretion and star for-mation processes. In addition high-frequencyreceivers will allow us to conduct (high spatial res-olution) follow-up observations and monitoringexperiments of AGNs, GRBs and other transientevents in connection with high-energy experiments(FERMI, MAGIC, CTA), a research ¯eld where theItalian astronomical community is very active.

Due to an agreement between INAF and ASI onthe use of the instrument for space applications,SRT will also be involved in planetary radar as-tronomy and space missions (Tofani et al., 2008;Grue® et al., 2004).

This paper presents an overview of the SRTsystem in the light of the results obtained during thecommissioning phase (Ambrosini et al., 2013). Sec-tion 2 describes the overall SRT system, includingtechnical speci¯cations of the main antenna modules.Section 3 shows the results of the tests performed inthe commissioning phase and the timeline for themain milestones. In Sec. 4 some conclusions aredrawn on the overall results of the commissioning.

For the convenience of readers, Table 1 provides aglossary of many acronyms used throughout the paper.

2. The Sardinia Telescope System

2.1. Antenna technical characteristics

The antenna design, whose schematic view is shownin Fig. 2, is based on a wheel-and-track con¯gura-tion. The main re°ector (M1) consists of a back-structure that supports — through actuators —the mirror surface, itself composed of rings ofre°ecting panels. A quadrupod, connected to theback-structure, supports the sub-re°ector (M2) andthe primary focus positioner and instrumentation.The main and secondary mirrors and the quadrupodlie on the alidade, which is a welded steel structure

Table 1. Glossary of the terms used.

Acronym Description

ACS Alma Common SoftwareACU Antenna Control UnitASI Agenzia Spaziale ItalianaAV Astronomical ValidationBWG BeamwaveguideEPTA European Pulsar Timing ArrayDBBC Digital Baseband ConverterDFB Digital Filter BankDiFX Distributed Fourier spectrum Cross-multipliedDISCOS Development of the Italian Single-dish Control

SystemDSP Digital Signal ProcessingEER Elevation Equipment Roome-VLBI Real-time VLBIEVN European VLBI NetworkFEM Finite Element MethodFOV Field of ViewHPBW Half-Power Beam WidthIF Intermediate Frequency bandINAF Istituto Nazionale di Astro¯sicaIRA Istituto di RadioastronomiaIRIG-B Inter-Range Instrumentation Group, time code

format BLCP Left Circular PolarizationLEAP Large European Array for PulsarOAA Osservatorio Astro¯sico di ArcetriOAC Osservatorio Astronomico di CagliariOTF On-The-FlyPFP Primary Focus PositionerPSD Position Sensing DeviceRCP Right Circular PolarizationRFI Radiofrequency InterferenceROACH Recon¯gurable Open Architecture and

Computing HardwareSDI Single-Dish ImagerSRT Sardinia Radio TelescopeT&F Time and FrequencyVLBI Very Long Baseline InterferometryWVR Water Vapor RadiometerXARCOS Arcetri Cross-Correlator Spectrometer

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standing on a large concrete tower that forms thebulk of the antenna foundation. Three large roomswere built behind the primary mirror to contain thesecondary focus receivers, the beam-waveguidemirrors and several electronic instrumentation andcable distributions, respectively. A fourth room islocated in the lower part of the alidade, where thepower drivers for the motors, the antenna controlunit and the cryogenic compressors are installed. Thealidade also supports the elevation wheel, which is aconical truss anchored to the re°ector back-structurethrough a massive pyramidal structure.

The radio telescope is supported by a rein-forced-concrete foundation excavated into rock; anouter ring beam sustains the azimuth track, while acentral building accommodates the azimuth pintlebearing support, the azimuth cable wrap and theencoder system. The wheel-and-track structure con-sists of 16 wheels lying on the rail, which is a contin-uous welded ring 40m in diameter. This is connectedto the foundation by 260 pairs of anchor bolts and bya ¯ber-reinforced grout. The overall structure of theantenna weighs approximately 3300 tons.

The antenna is steerable around the azimuthand elevation axes. The rotation around the twoaxes is managed by a servo control system, con-sisting in 29-bit absolute encoders (peak-to-peakposition error 0.8 arcsec), 12 brushless asynchronous

motors (eight in azimuth and four in elevation)and an ACU available on a BECKHOFF hardwareplatform employing an IRIG-B generator. A propertorque bias is applied to the motors to overcome thegearbox backlash and to improve the antennapointing accuracy.

The primary re°ector surface consists of 1008individual aluminum panels divided into 14 rows ofidentical panel types. Each panel has an arearanging from 2.4 to 5.3m2. It is built using alumi-num sheets glued, by means of a layer of epoxyresin, to both longitudinal and transversal Z-shapedaluminum sti®eners. The basic back-structure iscomposed of 96 radial trusses and 14 circumferentialtrussed hoops supported by a large center hub ring.The sub-re°ector surface consists of 49 individualaluminum panels with an average area of about1m2, whereas its back-structure is formed by 12radial trusses and three circumferential trussedhoops supported by a center hub ring. Three ofthese trusses are directly connected to a triangularsteel frame, which has the function of a transitionalstructure to the six sub-re°ector actuators. Theseactuators de¯ne the sub-re°ector position, andprovide for sub-re°ector motion with ¯ve degrees offreedom.

Further mirrors beneath the Gregorian focus,arranged in a BeamWaveGuide con¯guration, allow

Fig. 2. SRT: mechanical structure. The radio-frequency re°ecting surfaces are highlighted in red.

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the addition of four more focal points with magni¯edand de-magni¯ed F=D ratio in the intermediatefrequency bands (Fig. 3).

Two of the four designed BWG layouts havealready been constructed. They use three mirrors:M3 as a shared mirror, M4 for the ¯rst layout andM5 for the second one. The re-imaging optics forBWG layout I were designed for maximum focalratio reduction (right side of Fig. 3); whereas BWGlayout II was designed so that the output focal pointF4 lies beneath the elevation axis of the antenna(left side of Fig. 3). By an opportune rotation of M3,which takes 1.5min, the desired BWG layout isselected. All the mirrors are portions of ellipsoidsand present quite large apertures, as shown inTable 2. The two missing BWG layouts will beadded in a later stage to the present con¯gurationusing two more mirrors and an appropriate rotationof M3; they will be dedicated to Space Scienceapplications.

A rotating turret (Gregorian positioner assem-bly) is mounted eccentrically on the focal plane ofthe antenna dish, and can house eight separatecryogenic receiving systems and the associated feedhorns for operating up to 115GHz. A drive systemcan rotate the turret (within 2min for a completerotation of 340") so that any of the feed horns canbe positioned on the focal plane. The servo controlsystem consists of two brushless servo motors withdrivers and a position-control computer.

A mechanism selecting among di®erent recei-vers exists also in the primary focus. Several receiverassemblies, the exact number depending on theirdimensions, can be allocated at the secondary mir-ror back-structure side. An arm controlled by twoservo motors can, in less than 4min, place the re-quired receiver box in the primary focus position(Fig. 4), which can accommodate low-frequencyreceivers whose dimensions are not mechanicallycompatible with the Gregorian positioner assembly.

The remote and automatic control of all thesemovements makes the antenna available in fre-quency agility, switching among all the observingbands in a fast and unmanned way.

Suitable atmospheric conditions (see Table 2 forthe environmental speci¯cations) together with theactive surface will extend the use of the instrumentup to 100GHz. This capability also requires anadvanced metrology system for accurate antennapointing (see Sec. 2.3). The availability of a WaterVapor Radiometer (WVR) and a Weather Station,already in operation, will also allow the dynamicscheduling of the observations, further improvingthe telescope productivity.

A complex helium 5.5 (99.9995% pure gas)plant consisting of seven pairs of supply-and-returnlines assures that the microwave receivers, distrib-uted in the various positions, can operate at cryo-genic temperatures. The system is dimensioned sothat each line can serve up to three microwavereceivers. The total length of the lines reaches1.5 km, and they are composed of both rigid and°exible tubes.

Fig. 3. Technical view of the Gregorian and BWG roomscontaining the Gregorian turret and the BWG mirrors, respec-tively.

Fig. 4. Technical view of the primary focus area. The primaryfocus positioner, containing the primary focus receivers, and there°ecting surface of the sub-re°ector are highlighted in lightblue and red, respectively.

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For the radio frequency links (Fig. 5), more than3 km of coaxial cables connect the focal positions to acentral point located in the elevation EquipmentRoom. The highest frequency useable with the co-axial cables is 4GHz, with a maximum attenuationof 15 dB (at 2GHz) for the longest path (100m) andwith a matching coe±cient better than #20 dB overthe whole frequency band. An optical ¯ber cabling,based on both single-mode and multi-mode ¯bers,was deployed within the radio telescope for digital

and analog data transmission. The optical ¯bers areresponsible for the connection of the radio telescopeto the external infrastructures. Some customizedoptical links allow the transport of the astronomicalsignal, in the IF frequency band (0.1–2.1GHz), fromthe EER to the remote control and data processingroom. This solution, thanks to a link gain of 0 dBassociated with a dynamic range better than thedynamic range of the receiver itself, ensures that theoverall speci¯cations of any receiver are maintained.

Table 2. Speci¯cations of the SRT.

Characteristics Details Value

Range of motion Elevation 5–90"

Azimuth 180" ' 270"

Slew rate Elevation 0.5"/sAzimuth 0.85"/s

Mirrors size M1 – Primary mirror 64m (axially symmetrical)M2 – Sub-re°ector 7.9m (axially symmetrical)M3 – BWG shared between layouts I and II 3:921( 3:702mM4 – BWG layout I 3:103( 2:929mM5 – BWG layout II 2:994( 2:823m

Number of individual panels M1 – Primary mirror 1008 (14 circular rows)M2 – Sub-re°ector 49 (3 circular rows)

Expected surface accuracy M1 panel ) 65 micron rms(El ¼ 45") in precision M1 alignment (photogrammetry; holography) 290; 150 micron rmsenvironment conditions M2 panel ) 50 micron rms

Back-up structure (with active surface) 0Actuator accuracy and linearity 15 micron rmsOther (meas. errors; thermal and wind e®ect) 49 micron rmsTotal without holography 305 micron rmsTotal with holography 178 micron rms

Pointing accuracy Normal conditions without metrology systems 13 arcsecPrecision conditions with metrology systems 2 arcsec

F/D F1 – Primary focus 0.33F2 – Gregorian focus 2.34F3 – BWG layout I focus 1.37F4 – BWG layout II focus 2.81

Frequency range Primary focus 0.3–20GHzGregorian focus 7.5–115GHzBWG foci 1.0–35GHz

Precision environment conditions Wind < 15 km/hSolar AbsentPrecipitation AbsentTemperature #10"C to 30"CTemperature drift < 3"C/hHumidity < 85%

Normal environment conditions Wind < 40 km/hSolar Clear skyPrecipitation AbsentTemperature #10"C to 40"CTemperature drift < 10"C/hHumidity < 90%

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In order to connect the radio telescope to theInternet, mainly to transfer data during real-timeVLBI observations, SRT will make use of a 10-Gbpslink infrastructure, currently under construction.

2.2. Microwave receivers and back-ends

Notwithstanding the telescope's enormous collect-ing area and its advanced systems, able to ensurehigh e±ciency at all the operating frequencies andover the whole elevation range, state-of-the-art mi-crowave receivers and digital and analog back-endsare necessary in order to perform breakthroughastronomical discoveries. The microwave receiverguides, ampli¯es, ¯lters and down-converts the in-coming astronomical signal re°ected by the mirrors.It consists of a cascade of di®erent microwave pas-sive and active components, some of them cooled tocryogenic temperatures. The receiver mainly de¯nesthe frequency bandwidth, the antenna illumination,the receiver noise temperature and the cross-polarpurity. The radio receiver group of INAF isresponsible for the design of the whole receiverchain, both from an electronic and mechanical pointof view, for the assembly and testing and ¯nally forthe integration in the radio telescope.

The ¯rst-light of SRT was obtained usingthe following three receivers: the L- and P-band

dual-frequency coaxial receiver for primary-focusoperations (Valente et al., 2010), the C-band mono-feed receiver designed for the BWG layout I (Nestiet al., 2010; Orfei et al., 2011; Poloni, 2010; Peveriniet al., 2011) and the K-band multi-feed receiver(seven corrugated feeds) for the secondary focus(Orfei et al., 2010).

For the very ¯rst astronomical tests, theC-band receiver was installed in the Gregorian focuswith a cone section added in front of the feed-hornto match the di®erent F/D numbers. This choicewas made to avoid possible BWG mirror misalign-ments in the ¯rst pointing tests of the antenna.

Table 3 describes the main technical char-acteristics of the receivers currently installed,together with those foreseen to be completed in thenext three years. For each receiver the table lists theobservable sky frequency bandwidth, the focalposition, the number of simultaneous beams in thesky (specifying the number of polarizations), anestimation of the expected antenna gain and of thesystem noise temperature when the antenna pointsto the zenith. Finally, the last column indicates thestatus of the receiver.

All the multi-feed receivers (S-, K- and Q-band)are equipped with a built-in mechanical rotator,whose aim is to prevent sky ¯eld rotation while thetelescope is tracking.

Fig. 5. Schematic view of the signal transfer lines from the front-ends to the back-ends.

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The W-band receiver, which was decommis-sioned by the Plateau de Bure radio telescopes op-erated by IRAM, is mainly aimed at testing the SRTactive surface, and at acquiring know-how in the3-mm band. However, even though it is a single-pixel, one-linear-polarization and narrow-instanta-neous-bandwidth (600MHz) receiver, its installationon the SRT could play a relevant role in the mm-VLBI network, signi¯cantly extending its baselines.

The front-end outputs are connected to theback-ends either by coaxial cables (in case they areplaced within the radio-telescope, in particular theTotal Power and XARCOS back-ends) or via opti-cal ¯ber in order to reach the distant control room.

Back-ends represent key elements to the successof the observations. They can be either general-purpose or tailored to speci¯c scienti¯c activities.All the back-ends listed in Table 4 were installedand are now either commissioned or under test atthe telescope (Melis et al., 2014).

2.3. Optical system and active surface

The SRT optical system is based on a quasi-Gre-gorian pro¯le with shaping applied to both theprimary and the secondary surfaces. The presentgeometry results from a trade-o® between two goals:minimizing the overall system noise temperature,

Table 3. Microwave receivers installed and under construction for the SRT.

ReceiverFreq range

[GHz]Focal

positionPixels (

polarizationsExpected antenna

gain [K/Jy]

Expected systemtemperatureat zenith [K] Status

L- and P- band coaxial feed 0.305–0.410 F1 1( 2 0.47–0.59 50–80 Commissioned1.3–1.8 1( 2 0.50–0.60 17–23

C-band mono-feed 5.7–7.7 F3 1( 2 0.64–0.70 24–28 CommissionedK-band multi-feed 18–26 F2 7( 2 0.60–0.66 40–70 CommissionedS-band multi-feed 2.3–4.3 F1 5( 2 0.76 54 Under constructionC-band (low) mono-feed 4.2–5.6 F4 1( 2 0.62–0.70 30–35 Under constructionX- and Ka-band coaxial feed 8.2–8.6 F1 1( 1 0.64 120 Under testing

31.8–32.3 1( 1 0.57 190Q-band multi-feed 33–50 F2 19( 2 0.45–0.56 45–120 Under constructionW-band mono-feed 84–116 F2 1( 1 0.34a 115 Under refurbishment

aWith a surface accuracy of 178 micron and the metrological systems in operation.

Table 4. Back-ends available at SRT. In brackets are given the applications currently under testing or re¯nement andplanned to be fully operative within 2016.

Back-end Main features and usage InputsMax input

IF band (GHz) Integrat. timeMax spectralchannels

Min spectralresolution (Hz)

Total Power Bandwidth selectable 14 0.1–2.1 1–1000ms — —

Attenuation selectableIF distributorContinuum

DFB ADCs, 10 bit resolution 4 0–1.024 0.1ms–4 s 8192 !100010 bit DSPPulsar(Spectro-polarimetry)

XARCOS Spectro-polarimetry 14 0.125 10 s 2048 !250DBBC 8 bit DSP 4 0.512 0.1–1 s 4096 125,000

VLBI(RFI monitoring)(Spectroscopy)

ROACH ADCs, 8 bit resolution 2 0.512 1 s 8192 62,500LEAP - Pulsar, array(Pulsar, single dish)(Spectroscopy)

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mainly due to spill over, and reducing the standingwave pattern (due to internal re°ections betweenthe primary and the secondary mirrors and thusdetrimental to wideband spectroscopic observa-tions) without excessively sacri¯cing the Field ofView available from the Gregorian focus (Cortès-Medellin, 2002). The shaped parabola–ellipse pair,in fact, provides a wider focal plane than fullyshaped re°ectors; the FOV's radius is 20 " at 10% ofaperture e±ciency loss. The design placed moreemphasis on reducing the standing wave than onmaximizing the aperture e±ciency.

One of the most innovative features of the SRTis the active surface that consists of 1116 electro-mechanical actuators mounted in the backupstructure, beneath the primary mirror panel cor-ners, and distributed along radial lines. Its ¯rst aimis to reshape the mirror to compensate for the re-peatable deformations due to gravity. Exploitingadvanced real-time measurements, it will also cor-rect for wind and thermal e®ects (Orfei et al., 2004).Each actuator moves either upward or downward atthe corners of four adjacent panels, in the directionnormal to the local surface, with a maximum strokeof '15mm. Such a large stroke was required inorder to achieve the second aim, i.e. to modify theshaped pro¯le back to a surface that is parabolicenough to increase themaximumoperating frequencyobservable from the primary focus (Bolli et al., 2014).A further application of the active surface is also torecover the manufacturing deformations of the sec-ondary mirror (Bolli et al., 2003).

In order to perform high frequency observationsthe surface accuracy must be around 150!m.The contractual requirement for the alignmentof the primary re°ector was 500!m with a \goal" of300!m. Successive accurate photogrammetry mea-surement campaigns have shown a lower limit in thetotal RMS of the primary mirror equal to 290!m at45" of elevation (Süss et al., 2012). These measure-ments, carried out in six di®erent elevation posi-tions, allowed the production of a look-up table,used to correct the gravity deformation with theactuators in an \open loop" con¯guration. In orderto further improve the measurements of the primarymirror pro¯le, microwave holography measure-ments will be implemented by using the hardwaresetup successfully tested at the 32-m Medicina radiotelescope (Serra et al., 2012).

Moreover, several solutions are being studiedand implemented so as to perform real-time

measurements of the re°ector deformations and themisalignments errors in the subre°ector position, withthe ¯nal aim of correcting them in a \closed-loop"strategy (Pisanu et al., 2012; Prestage et al., 2004):

(i) improved FEM analysis — to understand thee®ects due to gravity, temperature and windon the behavior of the mechanical structure ofthe antenna, and to guide the optimized designof the sensor systems;

(ii) inclinometers — to monitor the status of thetrack, the inclination of the azimuth axis andthe variations of the alidade deformation as afunction of the temperature;

(iii) position-sensing devices— to optically monitorthe lateral shift of the secondary mirror;

(iv) optoelectronic linear sensors — to measure thedeformations of the primary mirror;

(v) optical ¯ber range¯nders — to measure thedeformations of the legs of the quadrupod inorder to control the sub-re°ector position;

(vi) thermal sensors — to map the distribution oftemperature in the most sensitive areas of theradio telescope and predict the deformationsinduced by thermal gradients.

2.4. Antenna control software

The SRT consists of a large number of modules anddevices to be managed and controlled in a timelyand precise fashion. The many observing modesavailable must be commanded and harmonized withthe data acquisition and the recording of time-stamps and housekeeping data. The overall controlsoftware performing all of these actions is calledNuraghe, a software package exceeding half a mil-lion lines of code. It runs in the framework of theALMA Common Software (ACS, a distributed-object framework in turn based on CORBA), whichprovides a general and common interface for high-level software, hardware and other software partsabove the operating system.

For single-dish operations, various modes aresupported such as raster scan, On-The-Fly (OTF)cross-scan and mapping, sky dipping, positionswitching and focusing. The system is also able tooperate all the three Italian antennas under acommon interface (user interface and schedulingsystem), as well as to accommodate external appli-cations such as VLBI or pulsar observations.

All data coming from the integrated back-endsare stored in a FITS ¯le. The current release of

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Nuraghe (Orlati et al., 2012), which is written inCþþ and Python, enables the operator to controlmost of the subsystems — e.g. the antenna mount,the receiver chains, the back-ends, the active surfaceand the minor servo (Buttu et al., 2012).

2.5. Auxiliary instrumentation

We now discuss several associated ancillary activi-ties such as radio spectrum monitoring, weatherparameters measurement and time-frequency stan-dard distribution, as they are fundamental aspectsfor the optimal operation of the SRT.

A mobile laboratory is available at the site tocarry out dedicated measuring campaigns to moni-tor and identify Radio Frequency Interference(RFI) in the frequency range between 300MHz and40GHz (Bolli et al., 2013). Special attention is paidto the frequency bands observed by the ¯rst-lightreceivers and to those allocated by the Italian Fre-quency Allocation Plan to the Radio AstronomicalService (see Sec. 3.3).

A rather sophisticated ground-based set ofweather measuring instruments are deployed at theSRT site. Besides conventional sensors — ambientthermometer, hygrometer, wind speed/orientationmeasurer and barometer — a multi-frequency radi-ometer is employed. It generates real-time estimatesof di®erent microwave brightness temperatures,opacity, zenith water vapor and liquid content. Themeasurements conducted so far show a 40–45%probability of ¯nding an integrated water vaporcolumn density of less than 10mm in winter time.The absence of cloud cover can be found 50% of thetime in winter and 80% during summer; typical liq-uid water values range between 0.2 and 0.7mm.

In winter, the opacity at 22GHz is lower than0.15Np for 90% of the time (40% during summer).At higher frequencies, in the 3-mm band, the winteropacity is lower than 0.15Np for 35% of the time.

The Time and Frequency laboratory consists ofan active hydrogen maser producing the referencesignals for the receiver local oscillators, the 1 PPS(Pulse per second) timing for data acquisition andall the related equipments. A dedicated GPS re-ceiver derives the local clock o®set (as a di®erencefrom UTC GPS) for VLBI and provides the time tothe antenna for its pointing via an IRIG-B genera-tor (Ambrosini et al., 2011b).

3. Performance Test

3.1. The ¯rst radio source

SRT detected its ¯rst radio source in Summer 2012.The observation was performed with the C-bandreceiver, installed in the Gregorian focus at thetime. The main uncertainties at that time were theencoder alignments of the two main movement axes.Using the Moon, a wide and bright target, as a ¯rstreference radio source and moving the antenna insteps of a half beam-size in a cross-scan, we mea-sured the following encoder o®sets: þ1.3" in azi-muth and #0.5" in elevation. The successive OTFscans were performed across 3C218, chosen because,being not too far from the Moon on that day, itallowed us to use the just-measured \local" o®sets,in the absence of a pointing model. The antennaresponse is illustrated in Fig. 6; the noticeable pat-tern asymmetry is due to the employment of ¯xedoptics, observing far from the elevation of theirmechanical alignment (45").

Fig. 6. The ¯rst observed radio source, 2012 August 8.

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3.2. Pointing accuracy

The SRT software was designed to manage a com-plete and independent model for each receiver andeach focus. Three models were thus processed, ini-tially considering 11.3 0, 2.7 0 and 0.8 0 as referenceHalf-Power Beam-Widths (HPBWs) for the L-band,C-band and K-band receivers, respectively.

To achieve a preliminary pointing model, weapplied the encoder o®sets obtained during the ¯rstradio source observation and produced many rawmaps over bright and compact sources in order tomeasure the displacement of the optical axis withrespect to the ideal one. This procedure permitted thecreation of a basic set of parameters, which was usedas a basis for the pointing models for all the three foci.

As a next step to improve the pointing modelaccuracy, we performed several series of spot obser-vations (OTF cross-scans along the Horizontalframe) of selected pointing calibrators, each timemeasuring the pointing o®sets. In order to minimizethermal e®ects on the structure we usually performedthe observation during the night, in any case avoidingthe sunrise. Once the whole azimuth/elevation planein the sky was satisfactorily sampled, the pointingmodel polynomial was ¯t to the dataset (Maneri &Gawronski, 2002; Guiar et al., 1986). This processrequired several sessions to converge to an acceptablesolution, for which themodel residuals are required tobe lower than one tenth of the beam size.

The RMS residuals after the ¯t are presented inTable 5 and show considerably less than one tenth ofthe beam size up to 23GHz. The pointing perfor-mance at higher frequencies (the beam dimension willbe around 12 arcsec at 3mm) will take advantage ofmetrological systems (as discussed in Sec. 2.3).

3.3. RFI

Dedicated campaigns were carried out, both withthe mobile laboratory and through the telescope, in

order to detect and identify the interfering signals.Generally speaking, the RFI environment was foundto be fairly quiet in the C-band (especially above6400MHz) and the K-band (almost everywhere inthe band), while the P- and L- bands appeared morepolluted. Figure 7 shows the spectral acquisitionsperformed with the various SRT receivers. Thesescans were carried out spanning the whole azimuthrange, repeating such 360" circles for di®erent ele-vations (every 3" above the horizon). These chartswere obtained in \max hold" mode: all the instanta-neous acquisitions are overplot and, for each fre-quency bin, the maximum recorded value isdisplayed, thus the diagrams represent a sort of\worst case scenario" of the signals detected in theabove speci¯ed az–el ranges.

The L- and P-band environment is the most\polluted" at the site. Under these conditions, inorder to ¯nd a suitable frequency con¯guration forthe Total Power back-end, whose narrowest built-inband is 300MHz, we installed external tuneable(5% of the central frequency) band-pass ¯lters. ForL-band acquisitions, an e®ective compromise wasfound and we successfully observed between 1696.5and 1715.0MHz. No solution, allowing reliable andrepeatable measurements in the P-band, was found.Almost all the interfering signals were identi¯ed tobe self-generated by the apparatus installed in thetelescope. Among these devices we can mention theVOIP phones in Alidade and elevation EquipmentRooms, the XARCOS back-end, the encoders of thePFP, and some devices of the control electronics ofthe K-band receiver.

As concerns the C-band receiver, the generalpanorama was quite satisfactory; we discovered thepresence of only a few interfering signals, mainlyconcentrated in the lower part of the receiverbandwidth. The signals at 5900 and 6016MHz havebeen identi¯ed to be self-generated. The former isproduced by one of the local oscillators of the multi-feed system, the latter by the device that guaranteesInternet connection to the station through a satel-lite link. The RFI signals coming from externalsources were all due to digital links, and theyappeared to be much attenuated when observingsouthward.

The K-band turned out to be almost free fromRFI. All the identi¯ed polluting signals came fromexternal sources were related to ¯xed links for mo-bile operators. These signals can be practicallyneglected by avoiding known azimuth and elevation

Table 5. Pointing model residuals and beam size for the dif-ferent receivers.

Observed

center freq.

Pointing modelresiduals [arcsec]

Rec. [GHz] #az #el Beam size [arcmin]

L 1.705 7 7 11.12C 7.35 10.8 7.2 2.58K 23 3.9 3.2 0.805

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positions. The bands allocated to radio astronomicalservice (22–22.5 and 23.6–24.0GHz) were con¯rmedto be fully usable.

We emphasize that several interfering signalsturned out to actually be \self-RFI", i.e. they wereproduced by devices installed at the SRT site. Sincemost of those devices are going to be moved into aproperly shielded room in a very near future, weexpect that the RFI environment will greatlyimprove and we thus do not consider the acquisi-tions performed during the commissioning to berepresentative of the ¯nal site conditions. It mustalso be stressed, for the sake of the future users ofthe SRT, that spectral back-ends — and speci¯cmitigation techniques — are being developed inorder to cope with RFI and are expected to beoperative within 2016 (see Table 4).

3.4. BWG focus commissioning

3.4.1. System temperature

The C-band receiver measured system temperatureversus the elevation position is shown for bothpolarizations (LCP and RCP) in Fig. 8. Values wereobtained for every 10" of elevation in the 6.7–7.7GHz sub-band. The atmosphere opacity wasestimated by performing a skydip scan ($0 ¼ 0:014).

The single expected contributions to the systemnoise temperature are listed in Table 6. The sum ofthese elements (23.7K, 22.0K) is comparable to theobtained measures, 27 and 25K, respectively forLCP and RCP. The 2-K di®erence between the ex-perimental curves is consistent with the di®erentreceiver temperatures (Trx), whereas the slight dif-ferences between theoretical and experimentalvalues can be explained with a higher-than-predicted

Fig. 7. Spectral plots illustrating the RFI a®ecting the various receiver bands. These \max hold" plots show all the signals receivedwhile observing in the whole azimuth range, for several elevations above the horizon.

Fig. 8. Tsys measured in the band 6.7–7.7GHz. Circles: LCP.Diamonds: RCP.

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spill-over temperature (mainly coming from theBWG mirrors).

Table 6 includes an extra-noise term caused bythe cover of the Gregorian room, a 1-mm Te°on ¯lm1.5m in diameter, which protects the apparatusfrom the external environment. The ¯lm trade nameis Virgin PTFE, and the refraction index (not pro-vided as a function of frequency) declared by themanufacturer is 1.35, yielding a re°ection amount-ing to 4%. We measured the Tsys increase by tem-porarily removing the cover; results are summarizedin Table 7, where the average of the recordedincrements is shown for di®erent frequency bands.As shown in Table 7, such a contribution appears inthe C-band measurements, but it is de¯nitivelymore signi¯cant in K-band.

3.4.2. Gain curve

The e±ciency measurement and the gain curveswere produced exploiting the continuum back-end,performing cross-scans in the Horizontal frame onbright, point-like calibrators (Table 8). Atmo-spheric opacity was extrapolated from skydip scans.

This campaign was repeated in two phases,re°ecting two di®erent telescope con¯gurations.During the ¯rst phase, the main mirrors were ¯xedto the reference mechanical alignment. The secondsession was carried out when the active surface andthe sub-re°ector were aligned according to photo-grammetry, in order to compensate for the gravi-tational deformations that vary with elevation.

The antenna gain in the 7.0–7.7GHz bandis shown in Fig. 9 for the ¯xed optics. The plot

compares the theoretical values (triangles) with theexperimental measurements for the LCP channel(circles). The expected curve was obtained takinginto account both the receiver and the antenna ef-¯ciency parameters, including the surface e±ciencycomputed from the RMS of the mirrors, in the hy-pothesis that the optics were aligned at all the ele-vations (not achievable in real measurements, as thesubre°ector was not tracking). The trend of themeasured curve re°ects the expected one, however again o®set (about 10%–15%) is evident at all ele-vations — including 45", where the antenna opticswere supposed to be e®ectively aligned.

The plots of Fig. 10 show the BWG receivergain curves for both the circular polarizations,obtained with active surface and subre°ectortracking. The °atness of the curves along all theelevation range demonstrates a great improvementdue to the compensation for structural deforma-tions; on the other hand, the peak is slightly belowexpectation (0.61 instead of 0.66K/Jy). We might

Table 6. Contributions to the Tsys in the 6.7–7.7GHz band.

Contribution Notes

Trx LCP 8.5KRCP 6.8K

Lab measurements

Gregorian cover 1.4K ObservationsAtmosphereþ CMB 4.1K $ ¼ 0:014 at zenithGround spill-over 2.7K SimulationBWG spill-over 7.0K Simulation

Table 7. Tsys contribution by the Gregorian room cover.

Cover, Tsys contribution (K)

6.7–7.7GHz 18–20GHz 20–22GHz 22–24GHz 24–26GHz

1.4 11.2 9.2 8.3 9.8

Table 8. List of the °ux density calibrators used for C-bandmeasurements (Ott, 1994).

Source RA J2000 Dec J2000Flux density at7.35 GHz [Jy]

3c48 01:37:41.2971 þ33:09:35:118 3.79753c147 05:42:36.1379 þ49:51:07:234 5.29113c286 13:31:08.2881 þ30:30:32:960 5.70713c309.1 14:59:07.578 þ71:40:19:850 2.34333c295 14:11:20.6477 þ52:12:09:141 4.05883c161 06:27:10.096 #05:53:04.72 4.3614

Fig. 9. Gain with mechanically aligned mirrors: measurements(red circles) and expected values (triangles).

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then conclude that the optical alignment of themain and secondary mirrors is quite satisfactory forthis wavelength, and that the employment of theactive surface is able to deliver an almost constantgain.

3.4.3. Beam size

Figure 11 shows the beam deformation (") withrespect to the estimated HPBW (beam size, %e), forboth telescope con¯gurations (¯xed and active sur-face). The beam size is a reliable indication of how

the optics of the telescope re°ect the theoreticaldesign; accordingly, the deformation of the beamalong the elevation span could be a symptom of a°awed telescope structure. The beam deformation iscomputed through Eq. (1), where %az and %el are thebeam sizes measured along the azimuth and eleva-tion axis, respectively. The comparison of the twocurves led us to the same conclusion inferredthrough the gain curves: the e±ciency reductionobserved with the ¯xed alignment is almostcompletely recovered using the active surface andthe tracking subre°ector.

" ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið%az # %eÞ2 þ ð%el # %eÞ2

p: ð1Þ

3.5. Gregorian focus commissioning

The characterization of the K-band receiver, locatedin the Gregorian focus, was obviously complicatedby the impact of weather conditions on radiation atthese wavelengths. It was thus decided, so as toachieve very accurate and e®ective measurements,to limit the commissioning activities to the timeintervals showing low and stable opacity. Duringthese clear-sky periods the opacity at the site turnedout to be particularly favorable.

3.5.1. System temperature

The K-band receiver Tsys measurements were per-formed following the same procedures employedfor the BWG receiver, for both the polarizations,repeating the procedure for each 2-GHz sub-band.Figure 12 shows the LCP values for the central feed.

Fig. 10. C-band measured gain curves with the active surfaceand the sub-re°ector movement operating: LCP (top), RCP(bottom). Dashed lines indicate the expected values in an idealscenario.

Fig. 11. C-band beam deformation. Squares: with ¯xed optics.Circles: with active surface and tracking subre°ector(# ¼ 1:45 arcsec).

Fig. 12. Tsys versus elevation measured in 2-GHz sub-bands,LCP of central feed. The zenith opacity values are also reported.

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The atmospheric contribution was evaluated bymeans of skydip scans, whose results were comparedto the radiometer measurements. Table 9 lists themain Tsys contributions.

Even taking into account the extra systemtemperature contribution due to the cover ¯lm, we¯nd that the measured values do not match expec-tations. The sum of these contributions reaches49K, quite distant from the observed value of 73Kif we refer to the 18–20GHz band. The cause of sucha di®erence between measured and theoretical wasnot completely understood and therefore is stillunder investigation.

3.5.2. Gain curve

The list of calibrators observed during the e±ciencymeasurements is given in Table 10. Each calibratorwas sampled after performing preliminary cross-scans to focus the telescope and achieve an opti-mized pointing. The atmospheric opacity was againestimated by means of skydip acquisitions.

A preliminary gain curve, achieved in the22–24GHz band, was acquired with the ¯xed opticscon¯guration. The experimental and the theoreticalresults, in Fig. 13, are comparable. Measurementsshow lower values with respect to simulations, asthe latter were computed taking into account con-stantly aligned — yet non-active — optics, whilemeasurements were carried out keeping the mirrorsin their reference positions, i.e. the positions

obtained with their mechanical alignment at 45" ofelevation. In this reference position, where theoptics alignment should be ideal, a slight loss in thepeak gain is still noticeable (0.52 versus 0.55K/Jy).This can be explained as a residual inaccuracy in themechanical alignment.

Similarly, the gain curves for both LCP and RCPwere measured enabling the active surface and thesubre°ector tracking. We expected the curve to be°at at 0.66K/Jy, taking into account the RMS of thesurfaces of mirrors, the alignment of M1— estimatedwith photogrammetry — and other involved para-meters such as: accuracy of the alignment of thesubre°ector panels, errors in the measurements onpanels, gravitational, thermal and wind e®ects onpanels, positioning accuracy of the actuators. Theplots in Fig. 14 clearly show that, in terms of peakgain, the data match the predictions. On the otherhand, even if the overall telescope e±ciency bene¯tsfrom the active surface and the tracking of the

Table 9. Tsys contributions in all the receivers sub-bands, measured with the central feed.

18–20GHz 20–22GHz 22–24GHz 24–26GHz Notes

Trx LCP 23.7K 24.4K 29.7K 33.9K Lab meas.Trx RCP 21.6K 18.0K 22.7K 27.1KGregorian cover 11.2K 9.2K 8.3K 9.8K ObservationAtmosph:þ CMB ($) 11.4K 12.7K 16.8K 14.7K El ¼ 90"

(0.040) (0.045) (0.060) (0.052)Ground spill-over 2.3K 0.6K 0.6K 0K Simulation

Table 10. Flux density calibrators, used for the K-band gaincurve (Ott, 1994).

Source RA J2000 Dec J2000Flux density at22.35GHz (Jy)

3c48 01:37:41.2971 þ33:09:35:118 1.20573c147 05:42:36.1379 þ49:51:07:234 1.76553c286 13:31:08.2881 þ30:30:32:960 2.4330NGC7027 21:07:01.593 þ42:14:10:18 5.3890

Fig. 13. Opacity-corrected gain curves for the K-band re-ceiver, with ¯xed optics. Theoretical values (triangles) werecomputed considering aligned mirrors at each elevation. Mea-surements (circles) were acquired with both the mirrors ¯xed inthe mechanically aligned positions.

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subre°ector, there is still a substantial decrease in thee±ciency below 45" of elevation. This excess of gainloss at lower elevations can easily be translated in asurface accuracy of about 500micron (El ¼ 20"). Ourinvestigations suggest that gravity deformationmeasurements were not su±ciently accurate at lowelevations. In view of the installation of higher fre-quency receivers, this problem shall be solved; how-ever, this will be done in the re¯nement phase byusing the microwave holography technique.

3.5.3. Beam size and bidimensional pattern

Measurements of the beam size and pattern wereperformed alternatively enabling and disabling theactive surface, in order to verify its e®ectiveness and toroughly evaluate the quality of the optical alignment.The outcome of these experiments was also meant tobe exploited to draw further conclusions about theobserved gain curves, and to prove whether there wasa relation between potential alignment inaccuraciesand the gain drop observed at low elevations.

Figure 15 compares the beam deformation "(Sec. 3.4.3, Eq. (1)), measuring the deviation from thenominal HPBW, observed in the two telescope con-¯gurations (¯xed and active surface) by means ofcross-scans on non-resolved sources. Figure 16,instead, shows two raw maps in arbitrary counts,acquired on one of these sources. They illustrate theantenna beam patterns, respectively obtained with

Fig. 14. Gain curve in the 22.0–22.7GHz band. LCP (top)and RCP (bottom) for the central feed, acquired enabling theactive surface and the subre°ector tracking. Dashed lines rep-resent the expected values in ideal conditions.

Fig. 15. Beam deformation, with respect to the ideal beam size,versus elevation. Squares: with ¯xed optics. Circles: with activesurface and subre°ector in tracking mode (# ¼ 2:25 arcsec).

Fig. 16. Two az–el raw maps of the same point-like source.Fixed optics (top) and active surface plus tracking subre°ector(bottom). Both for El > 60".

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the ¯xed and active mirrors, in the same elevationrange.

The telescope in ¯xed optics con¯gurationshowed a beam size rapidly deforming as thepointed elevation deviated from the mechanicalalignment elevation (45"). Astigmatism and comalobes, revealed in Fig. 17 (top), explain such a dis-tortion, at least for the upper range of elevation(El > 60"), that is compatible with the gain dropshown in Fig. 13. The pattern in Fig. 17 (top) showsalso a distortion tilting at about 45" with respect tothe az–el axes. This deformation is not aligned withthe gravity vector, so it must be a contribution ofseveral di®erent ones. The compensation of gravitydeformations obtained employing the active surfaceof the primary mirror, on the other hand, allowed usto obtain a particularly e®ective alignment for ele-vations above 45". Figure 17 (bottom) demon-strates that most of the distortions of the antennabeam pattern had disappeared in this range. Anasymmetry in the sidelobes is still visible, likely dueto some residual squint.

Finally, the map in Fig. 18, taken at the averageelevation of 22", highlights a signi¯cant asymmetryand deformation, visible both in the main beam andin the ¯rst sidelobe. This deformation can be con-sidered in agreement with the characteristics of thegain curve presented in Fig. 14, as it can beexplained by the current status of the optics.

3.6. Primary focus commissioning

3.6.1. System temperature

After the preliminary tests, the primary focus re-ceiver turned out to have an issue in the microwave

chain responsible for the signal phase shifting pro-ducing circular polarizations from the native linearones. The problem a®ected the RCP only: the noisecalibration signal was not detectable for this chan-nel. The cause, although immediately identi¯ed,could not be ¯xed before the completion of thecommissioning, thus no further measurements couldbe performed. For this reason, this paper discussesonly the results for the LCP.

Table 11 presents the theoretical contributorsleading to an estimated temperature of 27.0K whenthe telescope is parked at zenith and up to 44.0Kwhen the elevation angle is 5". Even considering allthe caveats, e.g. the intrinsic inaccuracy in theprediction of the noise calibration signal level — thesystem temperature is higher than expected possiblydue to an underestimation of the simulated spill-over and/or other noise sources. Figure 18 forexample provides hints about the in°uence of RFIon the measured Tsys values: it shows clusteredmeasurements, in particular in the elevation rangefrom 15" to 50", creating \jumps" in the Tsys trend.This likely derives from the presence of variable RFIa®ecting the signal level.

3.6.2. Gain curve

The L-band gain curve is given in Fig. 19. It showsslightly scattered measurements but, in this case,the gain value equal to 0.52K/Jy is entirely inagreement with the expectations (0.50–0.55K/Jyfor the band under test).

Fig. 17. Az–el raw map over a point-like source with activesurface enabled and tracking subre°ector (average elevation 22").

Fig. 18. System noise temperature versus elevation for theL-band primary-focus receiver (1.6965–1.7150GHz).

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These measurements were performed using theactive surface, yet only to re-shape the primarymirror in order to obtain a parabolic pro¯le. Ele-vation-dependent corrections were not applied, as itis not necessary to compensate for gravity defor-mations at these low frequencies. The parabolicpro¯le allowed us to increase the antenna gain withrespect to the original quasi-Gregorian shaping.Using the latter, the antenna gain would rapidlydecrease for frequencies above 1.3GHz, reaching0.4K/Jy at 1.7GHz. The re-shaping of the surfacemight allow us to pro¯tably employ receivers up toa frequency of 22GHz in the primary focus.

3.7. Summary of the commissioningmeasurements and milestones

The entire process of the telescope commissioningtook almost 18 months, from mid 2012 to the end of2013. Table 12 shows the schematic timeline of themajor steps carried out, including examples of thetechnical advancement that has been taking place inparallel to the AV activities.

Table 13 ¯nally summarizes the main telescopemeasured parameters.

Table 11. Tsys contributions (1.6965–1.7150GHz).

Contribution Notes

Trx LCP 13.0K Lab measurementsAtmosphereþ CMB El ¼ 90" 5.0K Observations and

El ¼ 5" 22.0K simulationsGround spill-over 9.0K Simulations

Fig. 19. Gain curve for the L-band receiver (1.6965–1.7150GHz).The dashed line indicates the expected peak gain.

Table 12. Main milestones of the commissioning and post-commissioning technical activities.

Task description Execution period

Antenna Software Interface tests 2012 JuneGregorian Feed Rotator and M3 Rotator commissioning 2012 JulyMinor Servo Software integration and veri¯cation 2012 July–2013 SeptemberGregorian focus receiver integration 2012 JulyFirst radio source with the C-band receiver 2012 AugustGregorian Focus commissioning 2012 August–2012 OctoberPrimary Focus Positioner and Subre°ector commissioning 2012 August–2013 SeptemberBWG Focus commissioning 2012 October–2012 DecemberPrimary Focus commissioning 2013 May–2013 JulyActive Surface Software integration and veri¯cation 2013 January–2013 SeptemberInauguration 2013 September 30Installation of back-ends (successive to the Total Power back-end) 2013 October–2014 DecemberUpgrade of the Nuraghe control system to comply with the AV needs 2015 June

Table 13. Summary of the telescope performance measurements for the ¯rst-light receivers. Tsys and peak gain are measurablewith an accuracy of 5% due to the intrinsic uncertainty in the temperature of the noise calibration signal.

Observed centerfreq. [GHz]

Beam size[arcmin]

Tsys, El ¼ 90" [K] ('5%) Tsys, El ¼ 30" [K] ('5%) Peak gain [K/Jy] ('5%)

Rec. LCP RCP LCP RCP LCP RCP

L 1.705 11.12' 0.11 34 n.a. 35 n.a. 0.52 (44.6%) n.a.C 7.35 2.58' 0.02 27 25 31 29 0.61 (52.4%) 0.60 (51.5%)K 23 0.805' 0.04 74 77 89 92 0.66 (56.6%) 0.65 (55.8%)

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4. Conclusions

During the technical commissioning phase, all thedevices foreseen for the telescope early activitieswere successfully tested. The SRT overall perfor-mance was proven to be close enough to expecta-tions. Three receivers (L-, C- and K-band) and aTotal Power back-end were fully characterized. Thenew antenna control system was continuously im-proved and updated during the activities, allowingfor the execution of test observations in the mostcommon single-dish modes. The capabilities of theprimary re°ector active surface and of the trackingsubre°ector were widely demonstrated, as themeasurements performed enabling these devicesturned out to be almost elevation-independent.Re¯nements regarding the optics are still neededonly for K-band observations; further improve-ments, also in anticipation of the installation of evenhigher frequency receivers (up to 100GHz), will beachieved utilizing microwave holography and me-trology techniques, at present under investigationand testing.

As concerns the environmental conditions of thetelescope site, our experiments con¯rmed that thelocation meets the requirements for high-frequencyobservations. The occurrence of RFI — aggravatedby the local presence of temporarily unshielded ap-paratuses — was found to be an impediment onlyfor P-band observations; the C band was mostlyemployable and the K band was con¯rmed to beparticularly unpolluted — especially above 20GHz.

The telescope was hence made available to theAstronomical Validation team, in charge of asses-sing its scienti¯c potentials, while proceeding withthe installation and testing of additional devices —such as digital spectrometers — in view of theshortcoming opening of the SRT to the worldwidecommunity.

Acknowledgments

The SRT project was funded mainly by the ItalianMinistry of Education, University and Research,joined by ASI and the Sardinian Regional Govern-ment, the latter supporting most of the local infra-structural activities. In the initial phases, theproject was managed by CNR, after which it waspassed to INAF. Three institutions belonging toINAF made the technical and managerial e®orts:IRA, OAC and OAA. The Nuraghe control systemis one of the creations of the INAF project named

DISCOS, aimed at providing the Italian radio tele-scopes with uni¯ed managing software.

The contract for the construction of the tele-scope structure was awarded to MT MechatronicsGmbH, Mainz, Germany.

The authors acknowledge Joseph Schwartz forhis valuable suggestions that improved the clarityand readability of the text.

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