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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)
AO4ELT - 24 June 2009 Slide 4
E-ELT Programme
Spot Elongation for Shack-Hartmann WFS
AO4ELT - 24 June 2009 Slide 5
h
b
2/ hbtelongation =
Central launch
Side launch
t
E-ELT Programme
Laser Launch Locations
AO4ELT - 24 June 2009 Slide 6
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
AO4ELT - 24 June 2009 Slide 7
E-ELT Programme
Atmospheric Parameters
Median seeing=0.8”r0=12.9cmT0=2.5msθ0=2.1”L0=25m
AO4ELT – 24 June 2009 Slide 8
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)
AO4ELT - 24 June 2009 Slide 9
E-ELT Programme
LGS Asterisms for GLAO + LTAO
AO4ELT - 24 June 2009 Slide 10
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)
AO4ELT - 24 June 2009 Slide 11
E-ELT Programme
GLAO side vs central launch
AO4ELT - 24 June 2009 Slide 12
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
AO4ELT - 24 June 2009 Slide 13
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
AO4ELT - 24 June 2009 Slide 14
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
AO4ELT - 24 June 2009 Slide 15
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)
AO4ELT - 24 June 2009 Slide 16
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
AO4ELT - 24 June 2009 Slide 17
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
AO4ELT - 24 June 2009 Slide 18
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
AO4ELT - 24 June 2009 Slide 19
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
AO4ELT - 24 June 2009 Slide 20
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