E-ELT Programme

Simulations of Laser Guide Star AdaptiveOptics Systems for the E-ELT

Richard Clare & Miska le LouarnESO

24 June 2009Paris, AO4ELT

E-ELT Programme

AO4ELT - 24 June 2009 Slide 2

Overview

GLAO +LTAO for the EELTOctopus simulatorLGS Wavefront SensingGLAO parametric study resultsLTAO parametric study resultsConclusions so far

E-ELT Programme

AO Systems considered in this talk

GLAO & LTAO simulations presented. Parametric studies inparallel with ATLAS (LTAO), MAORY (MCAO)Values used here not necessarily EELT baselinesEELT also considering MCAO & MOAO systems (not presented)

AO4ELT - 24 June 2009 Slide 3

E-ELT Programme

Octopus

Octopus is a parallelized end-to-end C code that runson the ESO cluster of ~100 machinesCommand matrix is maximum a posteriori (MAP)Phase screens generated with von Karman powerspectrum, assume frozen flow (Taylor)Modal reconstructionShack-Hartmann wavefront sensorsGaussian influence functions~10 hours to run 100 time steps for 1 case for GLAO on40 machines (this driven by large FOV/subap=14” andmassive pupil size)

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E-ELT Programme

Spot Elongation for Shack-Hartmann WFS

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h

b

2/ hbtelongation =

Central launch

Side launch

t

E-ELT Programme

Laser Launch Locations

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Central launch(all LGS from behind M2)

Side launch(all LGS from edge M1)

E-ELT Programme

Centroiding Algorithms

Matched Filter (Unconstrained)Correlation (with Center-of-Gravity withthresholding of correlation image)Center-of-GravityFor MF and CORR algorithms, referenceimages initially calculated from Gaussian basedon subaperture location relative to laser launchSubaperture images accumulated and whensufficient SNR these are used as referenceimages

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E-ELT Programme

Atmospheric Parameters

Median seeing=0.8”r0=12.9cmT0=2.5msθ0=2.1”L0=25m

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E-ELT Programme

LGS WFS Detectors

LGS detector study underway20x20 pixels per subapertureRead noise: 3e-/pixel/frame“Detectors for LGS WF sensing on an ELT:characteristics, special features, limitations,technology” J. KOLB, Thursday 1450Laser spot size FWHM =1.1” (for s=0.8”,z=30º)Chosen pixel scale=0.7”/pixel (~1.5pixels/FWHM)

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E-ELT Programme

LGS Asterisms for GLAO + LTAO

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Diameter 5.0 arcmin4 LGSs, 1NGS on-axis

LGS GLAO

5 in ring of diameter 1.5 arcmin+1 on-axis, NGS on-axis

6 LGS

LTAO

Asterism for simulations – not EELT baseline (different to ATLAS)

E-ELT Programme

Assumptions (caveats)

Sodium layer symmetrical (Gaussian) and staticHence no focus or “LGS aberrations”Rayleigh fratricide not included in thesesimulationsStrehl /EE calculated over (50/100) frames onlyAll subapertures are weighted equally inreconstructor regardless of elongation (cf C.Bechet talk this morning)

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E-ELT Programme

GLAO side vs central launch

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Parameters:84x84 LGS subapsEE @ 2.2µm (K band)

20x20 pixels3e read noise/pixel/frame500 Hz sampling frequency11.4 km Na FWHM1.1” spot FWHM0.7”/pixelMatched filter algorithmLGS diameter 5 arcminEE over 0.2 seconds NGS 1x1 (tip/tilt only)NGS 1e4 phots/frameSeeing=0.8”

All subapertures weighted equally here so gain possible with noise-weighting

Side needs 1.5x as many photons as centralto achieve the same EE

E-ELT Programme

GLAO WFS algorithm comparison

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Parameters:84x84 LGS subapsEE @ 2.2µm (K band)Side launch20x20 pixels3e read noise/pixel/frame500 Hz sampling frequency11.4 km Na FWHM1.1” spot FWHM0.7”/pixel

LGS diameter 5 arcminEE over 0.2 seconds NGS 1x1 (tip/tilt only)NGS 1e4 phots/frameSeeing=0.8”

Matched Filter best over expected photon range

E-ELT Programme

GLAO temporal sampling frequency

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Parameters:84x84 LGS subapsEE @ 2.2µm (K band)Side launch20x20 pixels3e read noise/pixel/frame100k phots/subap/s11.4 km Na FWHM1.1” spot FWHM0.7”/pixelMatched filter algorithmLGS diameter 5 arcminEE over 0.2 seconds NGS 1x1 (tip/tilt only)NGS 1e4 phots/frameSeeing=0.8”

~500Hz is indeed optimal

All points have 100k phots/subap/s=200phots/subap/frame*500Hz

E-ELT Programme

GLAO atmospheric profiles

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Parameters:84x84 LGS subapsEE @ 2.2µm (K band)Side launch20x20 pixels3e read noise/pixel/frame500 Hz sampling frequency11.4 km Na FWHM1.1” spot FWHM0.7”/pixelMatched filter algorithmLGS diameter 5 arcminEE over 0.2 seconds NGS 1x1 (tip/tilt only)NGS 1e4 phots/frame

Factor of 2 improvement in EE seen for all 3 atmospheres

Note: all atmospheres have ~50% at lowest layer (47m)

E-ELT Programme

GLAO FOV & sampling (selected cases)

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Parameters:84x84 LGS subapsEE @ 2.2µm (K band)Side launch

3e read noise/pixel/frame500 Hz sampling frequency11.4 km Na FWHM1.1” spot FWHM

Matched filter algorithmLGS diameter 5 arcminEE over 0.2 seconds NGS 1x1 (tip/tilt only)NGS 1e4 phots/frameSeeing=0.8”

Fewer, larger pixels better at lower flux. More, smaller pixels at higher flux

E-ELT Programme

LTAO Strehl plot

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Parameters:84x84 LGS subapsStrehl @ 2.2µm (K band)Central launch20x20 pixels3e read noise/pixel/frame1 kHz sampling frequency10.0 km Na FWHM1.1” spot FWHM0.7”/pixelMatched filter algorithmLGS diameter 1.5 arcminStrehl over 0.05 seconds NGS 3x3 subapsNGS 1e3 phots/frameSeeing=0.8”

300-400 photons/subaperture/frame sufficient for LTAO

E-ELT Programme

LTAO LGS WFS WFE(nm) plot

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Parameters:84x84 LGS subapsStrehl @ 2.2µm (K band)Central launch20x20 pixels3e read noise/pixel/frame1 kHz sampling frequency10.0 km Na FWHM1.1” spot FWHM0.7”/pixelMatched filter algorithmLGS diameter 1.5 arcminStrehl over 0.05 seconds NGS 3x3 subapsNGS 1e3 phots/frameSeeing=0.8”

Wavefront error due to LGS noise only< 100nm

Subtracting Strehls from noise-free case can isolate WFE due to LGS noise only

E-ELT Programme

LTAO side vs central launch

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Parameters:84x84 LGS subapsStrehl @ 2.2µm (K band)20x20 pixels3e read noise/pixel/frame1 kHz sampling frequency10.0 km Na FWHM1.1” spot FWHM0.7”/pixelMatched filter algorithmLGS diameter 1.5 arcminStrehl over 0.05 seconds NGS 3x3 subapsNGS 1e3 phots/frameSeeing=0.8”

All subapertures weighted equally here so gain possible with noise weighting

~1.5x as many photons for side launch to achieve same Strehl as central launch

E-ELT Programme

Conclusions/ Future Work

GLAO requires of the order 100-200photons/subaperture/frameLTAO requires of the order 300-400photons/subaperture/frameSide requires 1.5x as many photons as centralMatched filter promising WFS algorithm

Look at weighting subapertures according toelongation/noise (as per Bechet with FRIM)Implement non-Gaussian sodium profiles inOctopus

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E-ELT Programme

Acknowledgements

The authors wish to acknowledge the funding ofthis work through:FP7 Preparatory activity WP 9000FP6 ELT Design Study

[conference / meeting] - 30 juin 2009 Slide 21