Download Trade Study Report: Fixed vs. Variable LGS Asterism

Trade Study Report:
Fixed vs. Variable LGS Asterism
V. Velur
Caltech Optical Observatories
Pasadena, CA
Outline
 Introduction
 Assumptions
 Observing scenarios and assumptions
 Narrow fields
 High red-shift galaxies (studied with d-IFU)
 Cost implications
 Optomechanical
 RTC
 Conclusions
 Other issues raised by this trade study
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Introduction
 NGAO 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 Dictionary
 Consider 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
 Approach
 Evaluate the benefit based on WFE for two science cases[1]
(resource limited)
 Estimate cost impact at highest subsystem level (ballpark)
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Assumptions
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
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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 procedure
1.
2.
3.
Assume science target is on-axis (near central LGS)
For various sky coverage values, optimize system
performance to compare continuously variable and
discrete (5”, 25”, 50”) asterism radii
WFE vs. (off axis) TT star magnitude is plotted for the
case where the TT star is corrected using MOAO
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10% sky coverage case
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40% sky coverage case
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60% sky coverage case
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Observing 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 procedure
1.
2.
3.
Assume TT/TTFA field of regard is 30 arcsec and
brightest TT is mV = 17 (this corresponds to 10% sky
coverage).
Vary the science target position between 0” and 150”
from quincunx center
Optimize system performance to compare continuously
variable and discrete (5”, 25”, 50”) asterism radii
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High red-shift galaxies
900
0
800
20
40
700
60
600
80
500
100
400
120
300
140
200
160
100
180
0
200
0
10
20
30
40
50
Asterism Radius [arcsec]
WFE at science target [nm]
Continuous v. Discrete LGS Asterism Comparison
Opt Ast WFE
Discrete Ast W FE
Opt Ast Radius
Discrete Ast Rad
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Radius [arcsec]
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Example implementation for this study
(assumes telecentricity)
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Optomechanical 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
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RTC Implications
 Discrete asterism
 Requires reconstructors for the three asterisms,
updated according to changing Cn2(h)
 Continuous asterism
 Requires 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
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Conclusions
 Continuously 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
hardware
 Real-time and supervisory control software costs will
dominate
 Software costs allowing, we should assume
continuously variable asterism in the system
design
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Other 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 shifts
 LGS defocus due to Na layer density fluctuations.
 LGS HO WFS would benefit from a telecentric
optical space
 Chief ray always parallel to the optical axis
 Would save us the job of registering each WFS to the
DM as the asterism geometry changes
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References
1.
2.
R. Dekany, Private communication
R. Flicker , Private communication
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Backup Slides
HO WFS Positional Accuracy Tolerance:
WFE as a result of LGS deformation corresponding to “best
condition” narrow field case [2]
WFE [nm]
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Asterism radius [arcsec]
(w/ perfect asterism corresponding to lowest error)
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Stage costs
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5 ATS-1000 stages would be ideal for continuously
variable asterism.
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Newport stages
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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.
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