atlas the ltao module for the e-elt thierry fusco onera / dota on behalf of the atlas consortium
DESCRIPTION
Advanced Tomography with Laser for AO systems. ATLAS The LTAO module for the E-ELT Thierry Fusco ONERA / DOTA On behalf of the ATLAS consortium. LTAO. ATLAS. The ATLAS project. “Advanced Tomography with Laser for Ao Systems” Institute : ONERA, GEPI, LESIA - PowerPoint PPT PresentationTRANSCRIPT
ATLAS The LTAO module for the E-ELT
Thierry Fusco ONERA / DOTA
On behalf of the ATLAS consortium
Advanced Tomographywith Laser for AO systems
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Advanced Tomographywith Laser for AO systems The ATLAS project
• “Advanced Tomography with Laser for Ao Systems”
• Institute : ONERA, GEPI, LESIA
• Duration : 16 months in 2 phases• Phase 1 : 7 months (already done) • Phase 2 : 9 months
• Associated scientific instruments• HARMONI, • METIS,• SIMPLE,
Other potential users • MICADO, OPTIMOS
ATLAS
LTAO
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Advanced Tomographywith Laser for AO systems General Requirements for ATLAS
4m
1m
250mm
InstrumentM6
Geometry- ATLAS is a 4m diameter, 1m thick module.- Nasmyth focal plane is located inside ATLAS
Mass- ATLAS maximum mass is 2.5 tons (1.5 tons for the rotating structure plus 1 Ton for the supporting structure)
focal plane
Field derotation provided by ATLAS rotation
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Advanced Tomographywith Laser for AO systems ATLAS Error budget
• Specification : 50 (70%) @ K => 290 (210) nm rms
• LGS : 260 nm rms (goal = 170 nm rms)
• high order correction through tomographic process
• NGS : 125 nm rms (2 mas rms for TT)
• Fast tip-tilt correction (telescope windshake + turbulence)
• Slow measurement of high order modes (« truth sensor »)
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Advanced Tomographywith Laser for AO systems Laser Guide Stars
error budget
2NCPA
2errorn calibratio
2saturation DM
2lin wfs
2HF telescope
2tismanisoplana 3Dspot
2 varstructure sodium
2error model noise
2error model
2propag noise
2y tomographlgs
2chrom
2ref diff
2emp
2aliasing
2fit
2res
2
nC
t
• Deformable optics: M4 and M5 already “defined” – no possible optimization
• LGS number and positions
• LGS WFS design
• Control: Tomographic reconstruction Temporal effects RTC design
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Advanced Tomographywith Laser for AO systems LGS configurations (number & positions)
Optimum Baseline
6 LGS Baseline ~ 4.3’
No LGS beam overlap NGS patrol FoV Ø = 2’
3D parameter space (number position flux) Performance with 4 LGS << 5 LGS << 6 LGS
Small evolution with LGS FoV diameter
Patrol Fov Ø = 2 arcmin
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Advanced Tomographywith Laser for AO systems LGS : choice of a launching scheme
Fratricide effects
Launch behind M2
• Huge impact for some subapertures Rayleigh signal >> sodium one Useless sub-apertures Evolve with time (pupil rotation)
• Impact in nm rm ~ a few tens of nm rms to be consolidated
• Contamination of scientific instruments (HARMONI)
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Launch from M1 side
• No fratricide effects But :
Laser reconfiguration every TBC min/hours to avoid beam crosses
loop has to be open at these moments for TBC min (to be consolidated)
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Advanced Tomographywith Laser for AO systems LGS : choice of a launching scheme
Spot elongation and noise propagation
• Spot elongation and noise propagation
E2E simulation . Telescope = 21m. Scaling factors 6 LGS position : 1 min ring Representative of 42 m
Tomographic performance M1 ≡ M2
Even a small gain from a pure performance point of view !
More uniform propagation onto modes !
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Advanced Tomographywith Laser for AO systems LGS WFS concept
• 3 concepts are studying • SH WFS (various config)• YAW • Pyramid
• choice of a baseline SH
12x12YAW Pyr
4Q
Noise performance
Good Poor Good
Gain variations Good Bad Bad
Detector availability
Not yet COST COST
Sensitivity to RON
High Low Low
Baseline for phase A : SH 12x12Options (still under study) : 4Q & YAW
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Advanced Tomographywith Laser for AO systems Number of photons per sub-ap
• Assumption : SH-WFS 12x12 pixels
Noise propagation elongated < 2 x symmetric
Loop filtering => attenuation factor of 1.5
Sampling frequency : 500 Hz •
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Advanced Tomographywith Laser for AO systems Tomographic reconstruction
P = Turbulent layer altitudes & GS positionsM = WFS/DM model (IM)direct model Critical parameters !
Turbulent layer strength Regularisation term Less critical
WFS noise model Regularisation term Less critical
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Advanced Tomographywith Laser for AO systems Tomographic reconstruction
Altitude evolutionper layer
Strength evolutionper layer
Initial Cn² profile
Accurate knowledge on layer position is required especially for highest layer ( > 5 km) knowledge @ ± 250 m or less
Cn² strength is less an issue
Need of : Good Cn² profiler & identification procedure More data & more analysis !
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Advanced Tomographywith Laser for AO systems Laser Guide Stars
error budget
2NCPA
2errorn calibratio
2saturation DM
2lin wfs
2HF telescope
2tismanisoplana 3Dspot
2 varstructure sodium
2error model noise
2error model
2propag noise
2y tomographlgs
2chrom
2ref diff
2emp
2aliasing
2fit
2res
2
nC
t
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Advanced Tomographywith Laser for AO systems Requirements and Strategy
PERTURBATION
REQUIREMENTS
Strong WindShake (WS): 280 mas rms Turbulence : below WS/10 (in rms)
22222 125 rmsnmothersanisonoisetempo
On Tip/Tilt/FocusInt
KALMAN
Low magnitude GSLow signal rejection
500Hz
STRATEGY
Control optimization : Kalman Filter @ 500Hz Use of 2 NGS to perform tomography when there is no bright & close NGS
Increase sky coverage Optimization of the WFS spot size and energy
ADC (H & Ks bands) Dedicated local DM
• use of LGS data
• open loop correction (a la MOAO)
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Advanced Tomographywith Laser for AO systems Sky Coverage results
Nominal (Lo = 25m) Pessimistic (Lo = 50m)
Close to 100 % SC @ 60° Around 50 % SC @ Galactic pole
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Advanced Tomographywith Laser for AO systems Trade-off / possible simplifications
• Main constraint : deal with the telescope windshake at least 500 Hz of sampling frequency • Turbulence only required 100 to 200 Hz
• If the telescope windshake is reduced at the level of the turbulence
no more need of μDM probably no more need of ADC EXTREME SIMPLIFICATION OF THE NGS DESIGN
HIGHLY DEPENDS ON THE OUTER SCALE !!!!!!!!!!!
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Advanced Tomographywith Laser for AO systems Expected Performance
Optimization area
Possibility to “play” with the performance optimisation area -> best performance on axis -> optimisation in a given FoV
It just requires a matrix modification in the RTC
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Advanced Tomographywith Laser for AO systems Expected Performance
Comparison with other AO systems
AO systems SR on axis Sky Coverage@ Galactic pole
SCAO• Mag < 11• Mag < 12• Mag < 13.5
70 %55 % 35 %
<< 1 % (15” FoV)< 1 % (20” FoV) 1 % (30” FoV)
GLAO < 1 % 100 %
MCAO 46 %(average perf. over 53”x53”)
~ 50 %
LTAO 55 % ~ 50 %
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Advanced Tomographywith Laser for AO systems ATLAS performance : 100% SC
• Use of the “telescope” NGS for windshake estimation between 200 and 350 nm rms (assuming a 25 m outer scale and
a 0.71 arcsec seeing). This roughly leads to a final ATLAS performance in K band
(depending on the GS position from 5 -> 10 arcmin): SR = 0.6->0.5 %, FWHM = 15.5->16.9 mas, Jitter = 3.9->5.6 mas This value drops to SR = 0.4->0.2 %, FWHM = 20.9->33.1 mas, Jitter = 8.4->12.7 mas
• Use of 1 NGS magnitude 19 (in the patrol FoV [2’ Ø]) 87 % SC @ galactic pole 98.3 % SC for the whole sky Can be used for WS correction
Between 4 mas and 12 mas rms for TT Between 95 and 200 nm rms of defocus
SR : a few few tens of %