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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

NGAO Point and Shoot Trade Study Status

Richard Dekany, Caltech

Chris Neyman, Ralf Flicker, W.M. Keck Observatory

Caltech Optical Observatories

Presentation outline
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

Precision ao tip tilt criterion
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

Sky coverage limits for precision ao
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%




Caltech Optical Observatories

Includes some Keck assumptions

Moao for tip tilt sharpening for 100 sky
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

Performance improvement with moao sharpening
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

Lgs tomography for ngs sharpening
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

Tomography error components
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

Simulation assumptions
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

Lgs asterisms considered
LGS Asterisms considered:

Patrolling LGS

The inner Triangle geometry

was not optimized!

Caltech Optical Observatories


Simulation results 3a20 3a60
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

Simulation results 3a20 3a601
Simulation Results 3a20 + 3a60

NGS Field of Regard

60” Tip/tilt NGS

Contours are nmrms tomographyerror

Caltech Optical Observatories

Simulation results comparison
Simulation results comparison

Caltech Optical Observatories

Off pointing leads to improved ngs sharpening
Off-pointing leads to improved NGS sharpening

LGS ‘at the NGS’

~8” radial off-point

Caltech Optical Observatories

Patrolling lgs gains
Patrolling LGS gains

Caltech Optical Observatories

Some results of the pns study
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

More results of the pns study
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

Corollary results of the pns study
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

Some thoughts on ngs distribution
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

Phase implementation asterism options goal unchanged asterism upon expansion aka buildable
Phase implementation asterism options(goal: unchanged asterism upon expansion (aka buildable))


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)










Caltech Optical Observatories

Phase implementation asterism options goal buildable with flexible usage of minimal laser power
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








Caltech Optical Observatories

Practical concerns
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


  • 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