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NGAO Point and Shoot Trade Study Status

NGAO Point and Shoot Trade Study Status. Richard Dekany, Caltech Chris Neyman, Ralf Flicker, W.M. Keck Observatory. Presentation Outline. Sky coverage limits for precision AO MCAO vs. MOAO sharpening Simulation Results Fixed vs. patrolling LGS Optimum LGS patrol placement

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NGAO Point and Shoot Trade Study Status

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  1. NGAO Point and Shoot Trade Study Status Richard Dekany, Caltech Chris Neyman, Ralf Flicker, W.M. Keck Observatory Caltech Optical Observatories

  2. Presentation Outline • Sky coverage limits for precision AO • MCAO vs. MOAO sharpening • Simulation Results • Fixed vs. patrolling LGS • Optimum LGS patrol placement • Practical considerations • Corollary results from the PnS study • Some phased implementation options • Conclusions and Preliminary Recommendations Caltech Optical Observatories

  3. Precision AO tip/tilt criterion • 100% sky always correctable at some tip/tilt error • I’ll define a ‘precision tip/tilt criterion’ of 80% Strehl from residual tip/tilt errors • 1Dtilt = 0.225 /D Caltech Optical Observatories

  4. Sky coverage limits for precision AO • For sufficient laser power, LGS AO systems typically limited by tip/tilt errors based on NGS measurements • Classic AO • Seeing-limited visible tip/tilt guiding • Improved AO • AO sharpened, single near-IR tip/tilt/focus guiding • Next generation AO • AO sharpened, 3 near-IR tip/tilt/focus guiding • Tip/tilt tomography helps quite a lot • Ultimately • AO sharpened, multiple visible tip/tilt/focus guiding Prob(NGS; b=30) can provide SRTT(H) > 80% < 1% ~13% ~50% >95% Caltech Optical Observatories Includes some Keck assumptions

  5. MOAO for tip/tilt sharpening for ~100% sky • NGS patrol range for ~100% sky coverage measurement grows large • Precision tip/tilt criterion for Keck, wanting only 30% sky fraction requires 150” diameter patrol range • MCAO over a large field suffers generalized anisoplanatism • Optimal dual DM AO with Keck would yield <10% J-Strehl on NGS at 60” radius • MOAO can sharpen wide field NGS better than MCAO • < 100 nm rms MOAO implementation errors demonstrated by Gavel et al. (2008) Caltech Optical Observatories

  6. Performance improvement with MOAO sharpening Table 1. Benefit of MOAO sharpening compared to MCAO sharpening alone for a 60” off-axis field NGS, assuming the Mauna Kea Ridge atmospheric model and a 10-meter diameter telescope. These results pertain only to the wavefront error arising from the difference in how wavefront corrections are applied in the two paradigms. Both MCAO and MOAO approaches would additionally suffer tomography error from imperfect atmospheric sampling (§2). Caltech Optical Observatories

  7. LGS tomography for NGS sharpening • What is the best use of a certain (limited) number of LGS beacons, when tip/tilt errors are large? • We want to minimize tomography error in the NGS direction(s), while retaining good science direction tomography • Consider the case of an early phase Keck NGAO with a total of 6 sodium LGS beacons • Assume noise-free tomography for now • (Noise considerations are future work, but heuristic arguments and initial simulations show very little noise penalty - LGS photons contribute to science target tomography for any small metapupil shear.) Caltech Optical Observatories

  8. Tomography Error Components • Missing measurements (Type I) • Due to metapupil scale and shear • Turbulence height estimation error (Type II) • Applies when turbulence height is uncertain, even for a single thin turbulence layer • Unseen/Blind modes (Type III) • Applies when turbulence modes can combine to provide no WFS signal • Asterism uncertainty error (Type IV) • Due to tilt indeterminacy of each LGS Caltech Optical Observatories

  9. Simulation assumptions • LAOS software developed by TMT • Keck Telescope • 10-meter diameter • LAOS actuator spacing 0.35 m • Tomography error estimated by removing rms fitting error simulated with bright NGS • Evaluated over spatial grid of 49 points • Extrapolated to create contour plots estimating tomography error • Mauna Kea Ridge Median Turbulence Model (KAON #503) Caltech Optical Observatories

  10. LGS Asterisms considered: Patrolling LGS The inner Triangle geometry was not optimized! Caltech Optical Observatories arcsec

  11. Simulation Results 3a20 + 3a60 NGS Field of Regard 60” Tip/tilt NGS For even wider pentagon, azimuthal Variations worsen, resulting in lower Average NGS Strehl Contours are nmrms tomographyerror Caltech Optical Observatories

  12. Simulation Results 3a20 + 3a60 NGS Field of Regard 60” Tip/tilt NGS Contours are nmrms tomographyerror Caltech Optical Observatories

  13. Simulation results comparison Caltech Optical Observatories

  14. Off-pointing leads to improved NGS sharpening LGS ‘at the NGS’ ~8” radial off-point Caltech Optical Observatories

  15. Patrolling LGS gains Caltech Optical Observatories

  16. Some results of the PnS study • Revised WFE budget • Original KAON 429-based parametric ‘LGS density’ model overweighted the value of small asterisms • For sensible 4-9 LGS asterism, there appears to be no value of asterisms with less than 25” radius • Caveat: exact minimum pending off-zenith simulations • Uncovered cell error • Was applying IR sky bkgnd to HOWFS • Modified HOWFS error propagator model • Removed k1 from e = k1 + k2 ln(N2) model for LGS systems • Refactored HOWFS SNR calculation to better map onto LAOS Noise Equivalent Error (NEA) input schema Caltech Optical Observatories

  17. More results of the PnS study • Tomography error behavior • (Re-)Discovered the utility of a central LGS for sparse asterisms (N < 5-6) • Indicated by relatively large optimized radii for open-centered asterisms (ESO reported similar behavior at SPIE) • Correspondingly, for N > 5-6, a central LGS is not particularly beneficial • Noise behavior • Hypothesis is that PnS stars still contribute almost fully to the SNR of the science target wavefront estimate • Based on heuristic metapupil arguments (e.g. large overlap at 10km, even for 75” off-axis LGS) • So far, we’ve been unable to prove or disprove this using LAOS Caltech Optical Observatories

  18. Corollary results of the PnS study • Noise behavior of LAOS under investigation • N LGS of a given power yield measurement noise comparable to 1 LGS of that power • We need to run add’l ‘known’ cases to understand scaling behavior • Installed LAOS onto ~12 high-speed cores at Caltech • About a factor of 3-4 faster completion of future studies • Updated LAOS to newest version at WMKO • Once we’re up to speed, we hope to roll out to all machines Caltech Optical Observatories

  19. Some thoughts on NGS distribution • Due to computing overheads, we focused on a particular case of a wide-equilateral NGS asterism • Real NGS will be selected from distributions… • In angular separation away from the science target • The statistics of this can probably be worked out analytically • In brightness • More SNR may not benefit bottom line performance • Because PnS mostly benefits off-axis NGS, consider cost savings from 2 PnS stars (n.b. ‘dual wield’ or ‘akimbo’) • We could explore the performance gain of a single PnS LGS Caltech Optical Observatories

  20. Phase implementation asterism options(goal: unchanged asterism upon expansion (aka buildable)) PnS Tetrad (4) - opt. tomo vs. TT errs One on-axis + 3 PnS (4) Tetrad + 2/3 PnS (6/7) Tetrad + Triangle + 2/3 PnS (9/10) 1 2 1 2 PnS 1 2 1 PnS Caltech Optical Observatories

  21. Phase implementation asterism options(goal: buildable with flexible usage of minimal laser power) Triangle + 2 PnS (5) - one PnS could be put on-axis depending on 0 Pentagon + 2 PnS (7) - one PnS could be put on-axis depending on 0 1 PnS 2 2 1 PnS 1 Caltech Optical Observatories

  22. Practical concerns • Optomechanical complexity • Uplink and downlink (but not worse than dIFS anyway) • One instrumentation rule of thumb • $150K per (ambient T) mechanism • Implies • $300K for 2 DoF Point and Shoot • $900K for 6 re-deployable beacons (too dear) • Reconstructor generation • Need to pre-compute or rebuild reconstructors rapidly • Seems like a $200K-ish issue, but may be needed anyway… • Observational efficiency • Acquisition overhead not bad (LGS fast compared to faint NGS) • Sequencer / system complexity • Perhaps adding 5% to I&T costs? (another $400K?) Caltech Optical Observatories

  23. Conclusions • MOAO sharpening of NGS can benefit low-order sensing for NGAO • Upper limit to performance (median seeing) • TT NGS sensitivity gain • ~18% Strehl (absolute) in J • ~11% Strehl (absolute) in H • Bottom-line science target gain • Typically 4-10% J Strehl (absolute) depending on limiting errors • NGAO cost increment of PnS • Remains only very coarsely estimated • +$550-750K (1 PnS), +$700-900K (2 PnS), +$850-1,050K (3 PnS) • Preliminary Recommendations (Dekany only opinion) • Baseline 2 PnS LGS for now • Investigate 3, 2, 1 PnS options using TMT sky coverage code • Develop better cost estimate (particularly outside optomech) Caltech Optical Observatories

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