ATLASThe LTAO module for the E-ELT

Thierry FuscoONERA / DOTA

On behalf of the ATLAS consortium

Advanced Tomographywith Laser forAO systems

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Advanced Tomographywith Laser forAO systemsThe 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 forAO systemsGeneral Requirements for ATLAS

4m

1m

250mm

InstrumentM6

Geometry- ATLAS is a 4m diameter, 1m thickmodule.- Nasmyth focal plane is located insideATLAS

Mass- ATLAS maximum mass is 2.5 tons (1.5tons for the rotating structure plus 1Ton for the supporting structure)

focal plane

Field derotation provided by ATLAS rotation

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Advanced Tomographywith Laser forAO systemsATLAS performance requirements

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Advanced Tomographywith Laser forAO systemsATLAS 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 forAO systemsLaser Guide Stars

error budget

2

NCPA

2

errorn calibratio

2

saturation DM

2

lin wfs

2

HF telescope

2

tismanisoplana 3Dspot

2

varstructure sodium

2

error model noise

2

error model

2

propag noise

2

y tomographlgs

2

chrom

2

ref diff

2

emp

2

aliasing

2

fit

2

res

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 forAO systemsLGS configurations (number & positions)

Optimum Baseline

6 LGSBaseline ~ 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 forAO systemsLGS : 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 rmsto 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 toavoid beam crosses

loop has to be open at these moments for TBCmin (to be consolidated)

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Advanced Tomographywith Laser forAO systemsLGS : 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 !

9

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Advanced Tomographywith Laser forAO systemsLGS WFS concept

• 3 concepts are studying• SH WFS (various config)• YAW• Pyramid

• choice of a baseline

LowLowHighSensitivity toRON

COSTCOSTNot yetDetectoravailability

BadBadGoodGain variations

GoodPoorGoodNoiseperformance

Pyr4Q

YAWSH12x12

Baseline for phase A : SH 12x12Options (still under study) : 4Q & YAW

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Advanced Tomographywith Laser forAO systemsNumber of photons per sub-ap

• Assumption :SH-WFS 12x12 pixels

Noise propagationelongated < 2 x symmetric

Loop filtering=> attenuation factor of 1.5

Sampling frequency : 500 Hz•

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Advanced Tomographywith Laser forAO systemsTomographic 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 forAO systemsTomographic 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 !

4

Advanced Tomographywith Laser forAO systemsLaser Guide Stars

error budget

2

NCPA

2

errorn calibratio

2

saturation DM

2

lin wfs

2

HF telescope

2

tismanisoplana 3Dspot

2

varstructure sodium

2

error model noise

2

error model

2

propag noise

2

y tomographlgs

2

chrom

2

ref diff

2

emp

2

aliasing

2

fit

2

res

2

!!!!!

!!!!

!!!!!!!!

+++++

++++

++++++=

nC

t

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Advanced Tomographywith Laser forAO systemsRequirements and Strategy

PERTURBATION

REQUIREMENTS

Strong WindShake (WS): 280 mas rms

Turbulence : below WS/10 (in rms)

( )22222125 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 nobright & 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 forAO systemsSky 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 forAO systemsTrade-off / possible simplifications

• Main constraint : deal with the telescopewindshake

⇒ at least 500 Hz of sampling frequency• Turbulence only required 100 to 200 Hz

• If the telescope windshake is reduced at the level ofthe 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 forAO systemsExpected 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 forAO systemsExpected Performance

Comparison with other AO systems

~ 50 %55 %LTAO

~ 50 %46 %(average perf. over 53”x53”)

MCAO

100 %< 1 %GLAO

<< 1 % (15” FoV)< 1 % (20” FoV) 1 % (30” FoV)

70 %55 %35 %

SCAO• Mag < 11• Mag < 12• Mag < 13.5

Sky Coverage@ Galactic pole

SR on axisAO systems

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Advanced Tomographywith Laser forAO systemsATLAS 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 TTBetween 95 and 200 nm rms of defocus

SR : a few few tens of %

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Advanced Tomographywith Laser forAO systemsATLAS design : summary

• 6 LGS in 4.2 arcmin Ø (launch from M1 side)• SH WFS• 84x84 sub-aperture• 12x12 pixels per sub-aperture• Sampling freq: 500 Hz

• LGS / NGS separation with mirrors only• Pupil stabilisation by ATLAS rotation• LGS fix w.r.t telescope referential

• 2 arcmin natural guide star FoV• Patrol foV : 30”2’ Ø• Scientific FoV : 30”1’ Ø

• 2 low order NGS WFS• 2x2 SH-WFS• Pixel size : 15 mas• 500 Hz (windshake correction)• IR band (H-Ks) with ADC• Internal DM for “MOAO-like” correction (using LGS tomographic data)

• 1 high order NGS WFS• 84x84 sub-apertures• From 500 Hz (SCAO case) 0.1 Hz (truth sensor)• VIS band (with ADC)

One LGS arm With VCM

PERF (K band on axis) : 55 %SC (in H-Ks) : 50 % @ galactic pole

Potential issue : Size of M6 !!!

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Advanced Tomographywith Laser forAO systemsAtmospheric dispersion

Atmospheric Dispersion

0

500

1000

1500

2000

2500

450 650 850 1050 1250 1450 1650 1850 2050 2250 2450

Wavelength [nm]

Dis

pers

ion

[m

as]

Dispatm (15,700,0)

Dispatm (7.5,750,50)

Dispatm (0,800,100)

• 1600 – 1800 nm 30 mas (60°) / 10 mas (30°)• 1500 – 1800 nm 60 mas (60°) / 20 mas (30°)