Trade Study Report: Fixed vs. Variable LGS Asterism

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

2. 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

3. 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)

4. 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

5. Observing Scenario I: Narrow fields • Potentialadvantages 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 • 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 iscorrected using MOAO

6. 10% sky coverage case

7. 40% sky coverage case

8. 60% sky coverage case

9. Observing Scenario II: High red-shift galaxies • Potentialadvantages of variable asterism radius • Better tomographic (MOAO) correction of science target, assuming constant correction within the asterism and natural anisoplanatic fallout without • Evaluation procedure • 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

10. High red-shift galaxies

11. Example implementation for this study(assumes telecentricity)

12. 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

13. 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

14. 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

15. 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

16. References • R. Dekany, Private communication • R. Flicker , Private communication

17. Backup Slides

18. HO WFS Positional Accuracy Tolerance:WFE as a result of LGS deformation corresponding to “best condition” narrow field case [2] WFE [nm] Asterism radius [arcsec] (w/ perfect asterism corresponding to lowest error)

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

20. Newport 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.