Trade Study Report: Fixed vs. Variable LGS Asterism
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Transcript 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
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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 WFE
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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QuickTime™ and a
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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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