Trade Study Report: Fixed vs. Variable LGS Asterism V. Velur Caltech Optical Observatories Pasadena, CA
Dec 18, 2015
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
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
77
10% sky coverage case10% sky coverage case
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
88
40% sky coverage case40% sky coverage case
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
99
60% sky coverage case60% sky coverage case
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
1010
Observing Scenario II: High red-shift galaxiesObserving Scenario II: High red-shift galaxies
Potential advantages of variable asterism radius
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
Potential advantages of variable asterism radius
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)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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]
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Asterism radius [arcsec]
(w/ perfect asterism corresponding to lowest error)
WFE [nm]
2020
Stage costsStage costs
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
5 ATS-1000 stages would be ideal for continuously variable asterism.
2121
Newport stagesNewport stages
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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.