Shack-Hartmann tomographic wavefront reconstruction using LGS: Analysis of spot elongation and fratricide effect. Clélia Robert 1 , Jean-Marc Conan 1 , Damien Gratadour 2 , Thierry Fusco 1 ,Cyril Petit 1 , Jean-François Sauvage 1 , Nicolas Muller 1. 1 ONERA, 2 Obs. Meudon (LESIA). - PowerPoint PPT Presentation
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Shack-Hartmann tomographic wavefront reconstruction using LGS:
• Telescope diameter 21 m, 42x42 sous-pupilles, central occultation factor 0.3• No distorsion of Sodium profile•Images = elongated Gaussian, subap. FoV 10x10 arcsec^2, pixscale=0.75 “• Modal (KL) matrix-based MAP wavefront reconstruction with analytical WCoG • Tip/tilt LGS measurement, plane waves (!)
Sketches courtesy R. Myers
10 km LGS thickness -> 5 km,
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Downscaled simulation (1:2)•Telescope = 21m & 0.5 m subap
•6 LGS on 1 min ring (MAORY-like) •Medium LGS flux:
500 photons/subap/frame & 3 e- RON
Tomographic performance M1 ≡ M2
about 59 nmEven a small gain for edge launching
Edge launching gives more uniform propagation onto modes !
Impact of launching scheme: Central (M2) vs edge (M1)
[no fratricide effect]
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Spot elongation: launch from M1 side… why does it work?
Lowest elongation where the layer is seen only once
Courtesy M. Tallon & al.
Information redundancyfor large elongated spots
Side launchCentral launch
Schematic sketch with 3 LGSs
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performed by D. Gratadour (LESIA) based on Gemini code
code has been validated with experimental data (Gemini...)
common activity for MAORY / ATLAS / EAGLE studies
– Currently used for LGS tomography analysis (see next slides) – Will be used for Optimal LGS WFS algorithm definition & WFS design (correlation)
Modeling of fratricide background
Examples of fratricide effects21 m / 6 LGS (launch behind M2)
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MAORY-like case with fratricide
Downscaled simulation • Telescope = 21m & 0.5 m subap
• 6 LGS on 1 min ring •Medium LGS flux:
500 photons/subap/frame & 3 e- RON
rms error 20% smaller with edge launching
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Summary of fratricide effect impact
LGS
constellation ring diameter xx
MAORY
1 arcmin
ATLAS
2.1 arcmin
EAGLE
3.6 arcmin
Low
LGS flux
RON=0 e-
+ 47 nm
[+ 15 nm]
+46 nm
[+13 nm]
+47 nm
[+12 nm]
Medium LGS flux
RON=3 e-+37 nm
[+11 nm]
+38 nm
[+10 nm]
+36 nm
[+ 8 nm]
Downscaled simulation • Telescope = 21m & 0.5 m subaperure
• 6 LGS on xx arcmin ring
Quite uniform and moderate impact for each LGS asterism (in quadratic difference)Quite uniform and moderate impact for each LGS asterism (in quadratic difference)
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Impact of the number of reconstructed layers
ATLAS project
LGS asterism 4.2 arcmin
WFE (nm) 3 layers 5 layers
Centre with elongation
60 58
Edge with elongation
59 56
Without elongation
42 39
WFE stable with 10 reconstructed layers in a 10 m telescope simulation Impact of Cn2 profile uncertainties in altitude and strength ??
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Up to 32 m telescope simulation
• Fast & memory efficient developments for 42m simulations • Modal KL matrix-based MAP reconstruction, sparse matrices multiplication and storage• WFE still grows up in a 32 m telescope case: More unseen modes up to 2600 KL involved
Medium LGS flux, 2 reconstructed layers, with spider
Telescope diameter
10 m 16 m 21 m 32 m
Center 41 53 56 69
Edge 42 56 55 71
Not ellong. 31 39 38 49
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1. Development of a of a fast & memory efficient modal matrix-based MAPMAP reconstructor using “analytical” WCoG [1,2]
[1] Sandrine Thomas et al, MNRAS 2008, [2] Laura Schreiber et al, MNRAS 2009
2. Edge launching is better than central launchingRMS error 20% smaller when fratricide effect is accounted for
warning: LGS spot anisoplanatism neglected…
3. WFS noise model: “slope equivalent uniform noise”factor 2 reduction in noise variance wrt simplistic single LGS channel + not regularized reconstruction
even with relaxed requirement on photon flux (typically 500 ph/subap/frame with 3 e- RON)
Confirmed on 32m case
4. Pupil segmentation (spider, fratricide effect) has limited effect with regularized reconstruction (MAP)
Conclusion
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• Fast modal reconstructor development
• Spherical versus plane waves tomography & comparison with zonal E2E tool & Fourier codes (Cyril Petit presentation)
Fourier code update of MAORY / ATLAS / EAGLE projects (presentations of Diolaiti, Fusco, Rousset)
• Analysis of LGS spot anisoplanatism (phase and scintillation)[3] Scintillation and phase anisoplanatism in Shack-Hartmann wavefront sensing. Clélia Robert et al. JOSA A, Vol. 23, Issue 3, pp. 613-624 (2006).
Impact through tomographic reconstruction: see Nicolas Muller’s Poster
• Impact of Cn2 profile uncertainties in altitude and strength (presentations of Conan, Fusco)