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Adaptive Optics in the VLT and ELT eraAdaptive Optics in the VLT and ELT era

Wavefront sensors, Wavefront sensors, correctorscorrectors

François WildiObservatoire de Genève

Issues for designer of AO systemsIssues for designer of AO systems

• Performance goals:– Sky coverage fraction, observing wavelength,

degree of compensation needed for science program

• Parameters of the observatory:– Turbulence characteristics (mean and variability),

telescope and instrument optical errors, availability of laser guide stars

• AO parameters chosen in the design phase:– Number of actuators, wavefront sensor type and

sample rate, servo bandwidth, laser characteristics

• AO parameters adjusted by user: integration time on wavefront sensor, wavelength, guide star mag. & offset

Dependence of Strehl on Dependence of Strehl on and and number of DM degrees of freedomnumber of DM degrees of freedom

• Assume bright natural guide star

• No meas’t error or iso-planatism or bandwidth error

S exp 2 exp 0.28 d / r0 5 /3

r0 r0 0.5m / 0.5m 6 /5

S exp 0.28d

r0 0.5m

5 /30.5m

2

Deformable mirror fitting error only

Decreasing fitting error

• Assume bright natural guide star

• No meas’t error or iso-planatism or bandwidth error

Deformable mirror fitting error only

Reminder #1: Dependence of Strehl on Reminder #1: Dependence of Strehl on and number of DM degrees of and number of DM degrees of freedom (fitting)freedom (fitting)

Classical PIEZO actuatorsClassical PIEZO actuators

Large DM’s are on every ELT Large DM’s are on every ELT technological roadmaptechnological roadmap

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Existing MEMS mirror Existing MEMS mirror (sufficient for Hybrid-MOAO)(sufficient for Hybrid-MOAO)

Boston Micromachines 32x32 actuator, 1.5 um MEMS device.(In Stock)

Basics of wavefront sensingBasics of wavefront sensing

• Measure phase by measuring intensity variations

• Difference between various wavefront sensor schemes is the way in which phase differences are turned into intensity differences

• General box diagram:

Guidestar

Turbulence

Telescope Optics Detector Recon-structor

Wavefront sensor

Transforms aberrations into intensity variations

Computer

Types of wavefront sensorsTypes of wavefront sensors

• “Direct” in pupil plane: split pupil up into subapertures in some way, then use intensity in each subaperture to deduce phase of wavefront. REAL TIME– Slope sensing: Shack-Hartmann, pyramid

sensing– Curvature sensing

• “Indirect” in focal plane: wavefront properties are deduced from whole-aperture intensity measurements made at or near the focal plane. Iterative methods - take a lot of time.– Image sharpening, multi-dither– Phase diversity

Shack-Hartmann wavefront sensor Shack-Hartmann wavefront sensor concept - measure subaperture concept - measure subaperture tiltstilts

CCD CCD

f

Pupil plane Image plane

WFS implementationWFS implementation

• Compact

• Time-invariant

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CCD rapideCCD rapide

• CCD design complete

• 64 pins– 256x256 pixels– 1200 trames/s– < 1e bruit– Refroidissment Peltier

01 Dec 2005

PF Vis AO WFS 12

01 Dec 2005 PF Vis AO WFS 13

StoreArea

Image Area

240x12024□µm

StoreArea

Image Area

240x12024□µm

OP 1

OP 2 GainRegisters

OP 3

OP 4 GainRegisters

OP 8GainRegisters

OP 7

OP 6GainRegisters

OP 5

Metal ButtressedMetal Buttressed22ΦΦ 10 Mhz Clocks 10 Mhz Clocksfor fast image to for fast image to store store transfer rates.transfer rates.

8 L3Vision Gain 8 L3Vision Gain Registers/OutputsRegisters/Outputs..Each 15Mpix./s.Each 15Mpix./s.

Store slantedStore slanted to allow room for to allow room for multiple outputsmultiple outputs..

Split frame transfer 8-output Split frame transfer 8-output back-illuminated e2v L3Vision CCD for WFS.back-illuminated e2v L3Vision CCD for WFS.

TMT.IAO.PRE.06.030.REL02

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3. NGS WFS 3. NGS WFS

• Radial+Linear stages with encoders offer flexile design with min. vignetting

• 6 probe arms operating in “Meatlocker” just before focal plane

• 2x2 lenslets

• 6” FOV - 60x60 0.1” pix

EEV CCD60

Flamingos2 OIWFS