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

2

error 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

2

error 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/Focus

Int

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 %