Trade Study Report: Fixed vs. Variable LGS Asterism V. Velur Caltech Optical Observatories Pasadena,...

Trade Study Report: Fixed vs. Variable LGS Asterism

Trade Study Report: Fixed vs. Variable LGS Asterism

V. VelurCaltech Optical Observatories

Pasadena, CA

V. VelurCaltech Optical Observatories

Pasadena, CA

22

OutlineOutline

IntroductionAssumptionsObserving scenarios and assumptions

Narrow fieldsHigh red-shift galaxies (studied with d-IFU)

Cost implicationsOptomechanicalRTC

ConclusionsOther issues raised by this trade study

IntroductionAssumptionsObserving scenarios and assumptions

Narrow fieldsHigh red-shift galaxies (studied with d-IFU)

Cost implicationsOptomechanicalRTC

ConclusionsOther issues raised by this trade study

33

IntroductionIntroductionNGAO point design retreat

Identified the need for both narrow-field and wide-field asterism configurations

Natural question: “Should the point design have fixed or continuously variable asterism radius?”

WBS DictionaryConsider the cost/benefit of continually varying the LGS asterism

radius vs. a fixed number of radii (e.g. 5", 25", 50"). Complete when LGS asterism requirements have been documented

ApproachEvaluate the benefit based on WFE for two science cases[1]

(resource limited)Estimate cost impact at highest subsystem level (ballpark)

NGAO point design retreatIdentified the need for both narrow-field and wide-field asterism

configurationsNatural question: “Should the point design have fixed or

continuously variable asterism radius?”

WBS DictionaryConsider the cost/benefit of continually varying the LGS asterism

radius vs. a fixed number of radii (e.g. 5", 25", 50"). Complete when LGS asterism requirements have been documented

ApproachEvaluate the benefit based on WFE for two science cases[1]

(resource limited)Estimate cost impact at highest subsystem level (ballpark)

44

AssumptionsAssumptions

5 LGS’s in a quincunx 3 NGS’s randomly distributed

Field of regard varies with observing scenario

Two are TT sensors, one is a TTFA sensor

Other assumptions (atmospheric turbulence, noise, laser return, etc.) per NGAO June ‘06 proposal

5 LGS’s in a quincunx 3 NGS’s randomly distributed

Field of regard varies with observing scenario

Two are TT sensors, one is a TTFA sensor

Other assumptions (atmospheric turbulence, noise, laser return, etc.) per NGAO June ‘06 proposal

66

Observing Scenario I: Narrow fields

Observing Scenario I: Narrow fields

Potential advantages of variable asterism radius

Better tomographic correction of TT and TTFA stars provides better Strehl ratio on-axis for a given sky coverage

Evaluation procedure1. Assume science target is on-axis (near central LGS)2. For various sky coverage values, optimize system

performance to compare continuously variable and discrete (5”, 25”, 50”) asterism radii

3. WFE vs. (off axis) TT star magnitude is plotted for the case where the TT star is corrected using MOAO


Better tomographic correction of TT and TTFA stars provides better Strehl ratio on-axis for a given sky coverage

Evaluation procedure1. Assume science target is on-axis (near central LGS)2. For various sky coverage values, optimize system

performance to compare continuously variable and discrete (5”, 25”, 50”) asterism radii

3. WFE vs. (off axis) TT star magnitude is plotted for the case where the TT star is corrected using MOAO

77

10% sky coverage case10% sky coverage case

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

88




99




1010

Observing Scenario II: High red-shift galaxiesObserving Scenario II: High red-shift galaxies


Better tomographic (MOAO) correction of science target, assuming constant correction within the asterism and natural anisoplanatic fallout without

Evaluation procedure1. Assume TT/TTFA field of regard is 30 arcsec and

brightest TT is mV = 17 (this corresponds to 10% sky coverage).

2. Vary the science target position between 0” and 150” from quincunx center

3. Optimize system performance to compare continuously variable and discrete (5”, 25”, 50”) asterism radii


Better tomographic (MOAO) correction of science target, assuming constant correction within the asterism and natural anisoplanatic fallout without

Evaluation procedure1. Assume TT/TTFA field of regard is 30 arcsec and

brightest TT is mV = 17 (this corresponds to 10% sky coverage).

2. Vary the science target position between 0” and 150” from quincunx center

3. Optimize system performance to compare continuously variable and discrete (5”, 25”, 50”) asterism radii

1111

High red-shift galaxiesHigh red-shift galaxies

Continuous v. Discrete LGS Asterism Comparison

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60

Radius [arcsec]

WFE at science target [nm]

0

20

40

60

80

100

120

140

160

180

200

Asterism Radius [arcsec]

Opt Ast WFE

Discrete Ast WFE

Opt Ast Radius

Discrete Ast Rad

1212

Example implementation for this study(assumes telecentricity)

Example implementation for this study(assumes telecentricity)



1313

Optomechanical ImplicationsOptomechanical Implications

Discrete asterism Requires HO WFS positioner that is repeatable upon asterism

reconfiguration To change from a 5 to 50 arcsec quincunx we need 40 mm travel at the focal plane.

Continuous asterism Requires HO WFS positioner with higher accuracy The accuracy of getting LGS spots on the HOWFS is a combination of the

stage position and the amount of asterism deformation that we can tolerate

The minimum error allocated for LGS asterism deformation in the NGAO proposal is 5 nm

This corresponds to ± 0.1 arcsec change in radius of the entire asterism! The HO WFS positioner need only position to this accuracy2

This is ~70 micron accuracy over the necessary travel range. This assumes that the uplink tip/tilt (UTT) has 0.1 arcsec (Ball Aerospace has mirrors

can provide 0.001 arcsec on sky) resolution on sky. This loose tolerance would enable us to position the HO WFS continuously

without much difficulty.Stronger cost driver will probably be the required angular tolerances of

matching the incoming beam

Discrete asterism Requires HO WFS positioner that is repeatable upon asterism

reconfiguration To change from a 5 to 50 arcsec quincunx we need 40 mm travel at the focal plane.

Continuous asterism Requires HO WFS positioner with higher accuracy The accuracy of getting LGS spots on the HOWFS is a combination of the

stage position and the amount of asterism deformation that we can tolerate

The minimum error allocated for LGS asterism deformation in the NGAO proposal is 5 nm

This corresponds to ± 0.1 arcsec change in radius of the entire asterism! The HO WFS positioner need only position to this accuracy2

This is ~70 micron accuracy over the necessary travel range. This assumes that the uplink tip/tilt (UTT) has 0.1 arcsec (Ball Aerospace has mirrors

can provide 0.001 arcsec on sky) resolution on sky. This loose tolerance would enable us to position the HO WFS continuously

without much difficulty.Stronger cost driver will probably be the required angular tolerances of

matching the incoming beam

1414

RTC ImplicationsRTC Implications

Discrete asterismRequires reconstructors for the three asterisms,

updated according to changing Cn2(h)

Continuous asterismRequires reconstructors updated according to asterism

radius and changing Cn2(h)

Question is: How do you choose the asterism radius?Need some auxiliary process and/or measurement

Differential cost for estimating, setting, logging, and perhaps defending the choice of radius and corresponding reconstructor unknown

Discrete asterismRequires reconstructors for the three asterisms,

updated according to changing Cn2(h)

Continuous asterismRequires reconstructors updated according to asterism

radius and changing Cn2(h)

Question is: How do you choose the asterism radius?Need some auxiliary process and/or measurement

Differential cost for estimating, setting, logging, and perhaps defending the choice of radius and corresponding reconstructor unknown

1515

ConclusionsConclusionsContinuously variable asterism

There is little performance benefit in narrow field performance

There is significant performance benefit for d-IFU science when the mismatch between asterism and target radius exceeds 20 arcsec

There is little cost overhead in optomechanical hardwareReal-time and supervisory control software costs will

dominate

Software costs allowing, we should assume continuously variable asterism in the system design

Continuously variable asterismThere is little performance benefit in narrow field

performanceThere is significant performance benefit for d-IFU

science when the mismatch between asterism and target radius exceeds 20 arcsec

There is little cost overhead in optomechanical hardwareReal-time and supervisory control software costs will

dominate

Software costs allowing, we should assume continuously variable asterism in the system design

1616

Other important considerationsOther important considerations

There are many concerns pertaining to LGS HO WFS focus requirements

LGS (differential) defocus between the 5 beacons due to projection geometry

LGS defocus due to global Na layer shiftsLGS defocus due to Na layer density fluctuations.

LGS HO WFS would benefit from a telecentric optical spaceChief ray always parallel to the optical axis Would save us the job of registering each WFS to the

DM as the asterism geometry changes

There are many concerns pertaining to LGS HO WFS focus requirements

LGS (differential) defocus between the 5 beacons due to projection geometry

LGS defocus due to global Na layer shiftsLGS defocus due to Na layer density fluctuations.

LGS HO WFS would benefit from a telecentric optical spaceChief ray always parallel to the optical axis Would save us the job of registering each WFS to the

DM as the asterism geometry changes

1717

ReferencesReferences

1. R. Dekany, Private communication2. R. Flicker , Private communication1. R. Dekany, Private communication2. R. Flicker , Private communication

Backup SlidesBackup Slides

1919

HO WFS Positional Accuracy Tolerance:WFE as a result of LGS deformation corresponding to “best

condition” narrow field case [2]

HO WFS Positional Accuracy Tolerance:WFE as a result of LGS deformation corresponding to “best

condition” narrow field case [2]



Asterism radius [arcsec]

(w/ perfect asterism corresponding to lowest error)

WFE [nm]

2020

Stage costsStage costs



5 ATS-1000 stages would be ideal for continuously variable asterism.

2121

Newport stagesNewport stages





Depending on the WFS optical tolerances on the angle of the incoming beam, this could be as low as $10,000-

$20,000. The cost driver for the stages would be the WFS’s angular sensitivity.

Trade Study Report: Fixed vs. Variable LGS Asterism V. Velur Caltech Optical Observatories Pasadena,...

Documents

Transcript of Trade Study Report: Fixed vs. Variable LGS Asterism V. Velur Caltech Optical Observatories Pasadena,...