Caltech Optical Observatories1 NGAO Point and Shoot Trade Study Status Richard Dekany, Caltech Chris...

Caltech Optical Observatories 1

NGAO Point and Shoot Trade Study StatusNGAO Point and Shoot Trade Study Status

Richard Dekany, CaltechRichard Dekany, Caltech

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


Presentation OutlinePresentation 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


Precision AO tip/tilt criterionPrecision 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

SR1

1 2

2

1D tilt

D

2


Sky coverage limits for precision AOSky coverage limits for precision AO

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%

For sufficient laser power, LGS AO systems typically limited by tip/tilt errors based on NGS measurements

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)


Performance improvement Performance improvement with MOAO sharpeningwith MOAO sharpening

AO Mode for NGS sharpe ning

Error term RMS wavefront error at 60Ó off-

axis distance

J-Strehl upper limit

H-Strehl upper limit

MCAO alone Generalized anisoplanatism 301 nm 10% 27%

MOAO Go-to control errors (incl. calibration) < 100 nm >78% >87%

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).


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.)


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

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Tomography Error ComponentsTomography Error Components


Simulation assumptionsSimulation 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)

r0 = 16 cm and theta0 = 2.70 arcseconds


LGS Asterisms considered:LGS Asterisms considered:

-100

-50

0

50

100

-100 -50 0 50 100

50 asec PentagonalPacking

20 asec Triangle + 60asec Patrolling LGS(aka 3a20 + 3a60)

Patrolling LGS

arcsec

The inner Triangle geometry was not optimized!


Simulation Results 3a20 + 3a60Simulation Results 3a20 + 3a60

Contours are nmrms tomographyerror

NGS Field of Regard

60” Tip/tilt NGS

For even wider pentagon, azimuthalVariations worsen, resulting in lower

Average NGS Strehl


Simulation Results 3a20 + 3a60Simulation Results 3a20 + 3a60

Contours are nmrms tomographyerror

NGS Field of Regard60” Tip/tilt NGS


Simulation results comparisonSimulation results comparison


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

~8” radial off-pointLGS ‘at the NGS’


Patrolling LGS gainsPatrolling LGS gains

LGS A sterism RMS tomography

wavefront error in the three NGS directions

J-Strehl upper limit

H-Strehl upper limit

Pentagonal packing with 50Ó radius 110-140 nm

(125 nm mean) 67% 80%

Inner 20Ó radius triangle and three patrolling LGS pointed directly at field NGS

95 nm 80% 88%

Inner 20Ó radius triangle and three patrolling LGS at optimum location

80 nm 85% 91%


Some results of the PnS studySome 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


More results of the PnS studyMore 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


Corollary results of the PnS studyCorollary 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


Some thoughts on NGS distributionSome 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


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

1

1

1

2

2

2

1

PnS

PnS

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)


Phase implementation asterism optionsPhase implementation asterism options(goal: buildable with flexible usage of minimal laser power)(goal: buildable with flexible usage of minimal laser power)

2

1

11

PnS

PnS

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

2


Practical concernsPractical 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?)


ConclusionsConclusions

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 Observatories1 NGAO Point and Shoot Trade Study Status Richard Dekany, Caltech Chris...

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