Adaptive OpticsOverview

(Astronomical)

Richard MyersDurham University

William Herschel Telescope with GLASRayleigh Laser Guide Star

Photo: Tibor Agocs, Isaac Newton Group of Telescopes

Outline• Generic Astronomical AO System

– Specifying AO for AtmosphericTurbulence Compensation

– and where this is not necessarily validfor more general applications

– Astronomical AO Components• 2nd generation Astronomical AO modes

– Wide field AO• Multi-Conjugate AO• Ground Layer AO• Laser Tomographic AO• Multi-object AO

– Very high order correction• eXtreme AO

3

Generic Astronomical Adaptive Optics

Telescope

Science

target

Laser

Natural

Guide

Star

Corrected

Science

focus

dichroic

beamsplitter

IR

light

Visible

light

Wave-

front

Sensor

Adaptive

Mirror

Control

System

wavefront

information

control

signals

atmospheric

turbulence

*

*

Uncorrectedimage

CorrectedImage

Uncorrectedwavefront

Correctedwavefront

Correcting the fluctuating aberrations caused by atmospheric turbulenceabove ground-basedoptical and near-infraredtelescopes.

Basic SingleConjugate AO system

• Kinetic energy in large scale turbulence cascades to smaller scales• Inertial interval

– Inner scale l0 - 1cm. Outer scale L0-10 to 100 m

• Refractive index Structure Function for separation r :

The AtmosphereKolmogorov model of turbulence

J. Vernin, Universite de Nice. Cerro Pachon for Gemini IGPO

[ ]2

2 2 / 3

( ( ) ( )

( )

n

n n

D r n r n

D r C r

! !) = + "

=

• Fried parameter r0 (∝λ6/5):– Size of aperture where

• Diffraction width = Seeing width– For infinite outer scale Kolmogorov turbulence in the near

field, r0 and the telescope diameter D are the only parametersrequired to:

• derive image profiles• determine the number of Deformable mirror actuators required to

produce a given residual wavefront phase variance (on average)~(spacing/r0)5/3

• Determine the required interactuator stroke• But Cn

2(h) will strongly affect off-axis performance• and Scintillation (amplitude variation) is often important in non-

astronomical AO - worst case: phase branch points• Thermal blooming

5 / 3

0

3/ 52top atm

2

00

2D phase structure function at telescope for plane waves:

6.88

2where 0.423 sec ( )

and is the zenith angle

n

rD

r

r C h dh

!

" #$

#

%

& '= ( )

* +

& '& '= ( )( )* +* +

,

Laser Applications of AO

Types of Adaptive Mirror(J.C.Dainty)

Continuous Facesheet Deformable Mirrors

Laser Applications of AO

Bimorph Mirror(J.C.Dainty, Imperial College)

Laser Applications of AO

BimorphDeformable Mirror

Laser Applications of AO

The ELECTRA Segmented Adaptive Mirror(76 tip-tilt-piston segments)

built by ThermoTrex, San Diego

228 degreeof freedomadaptivemirror

Laser Applications of AO

Fitting Error for ContinuousFacesheet Deformable

Mirror (and segmented)Flexible continuous phase sheet

reflectivesurface Actuators:

typically PZT or PMNthrow: 2-20 microns

Minimum physicalactuator separation ~ 7mm

Fitting error:σ2

f =κ (rs/r0)5/3 rad2

Lots of Astronomical assumptions!rs= projected actuator separation on sky κ = fitting coefficient for DM type. (continuous face sheet: 0.35-0.4)

Deformable Mirror Actuators1st generation DMs all involved piezoelectric (PZT) /

electrostrictive (PMN) actuators:– Serious Hysteresis (typically 5-40% of full range)– Curie Point (rapid change of level of hysteresis with temp)– Often limited stroke (hence stacked actuators)– Drive voltage (+/- 400V for low hysteresis “hard” PZT)

• OR magnetostrictive or voice call actuators for higherstroke applications– Non-linearity, bulk, power

• Newer DMs are available with electrostatic, magneticand electromagnetic actuators– Electrostatic

• low hysteresis• MEMS construction (300-500 micron spacing)• 4K actuator devices available• But non-linear, stroke still limited (4 -6 microns mechanical)

– Magnetic• Essentially no hysteresis• Low temperature operation• High stroke• 0.5V operation (COTS CMOS!)

Boston MicromachinesMEMS deformable mirror

Raw: 148 nm RMS WFE Flattened: 6-12 nm WFE In-band: 0.6 nm WFE

32x32 MEMS Evans et al 2006 Optics Exp. 14 5558

Electrostaticallyactuateddiaphragm

Attachmentpost

Membranemirror

GPICourtesy: Bruce Macintosh, LLNL

4k MEMS prototype

• 64x64 MEMS prototypes now in testing• 4 micron stroke Surface quality: 10-30 nm

RMS surface finish, 2-4 microns PV overallcurvature

4k MEMS prototypeCourtesy: Bruce Macintosh, LLNL

Parameter Value Comments

Clear aperture disk diameter 40 mm ± 5 mm Range of acceptable D is 30mmto 100mm (TBC)

If this specification cannot be met, please advice. Itmight be possible to accept a D between 30 mm and100 mm (TBC).Difference in x and y: overall slightly elliptical shapemight also be required.

Number of actuators across thediameter of the clear aperture disk

N=64N=84N=112

Regular Cartesian array assumed.

Yield 100% in mirror clear aperture for D as definedin DM19

Actuator Spacing D/(N-1) mm For D and N see DM19and DM20 respectively

Actuator Geometry Square Might want to investigate the feasibility of having adifferent spacing in the x and y directions.

Actuator Stroke (PV) Larger than or equal to 6 µm P to V mechanicalstroke

Not including provisions the manufacturer may takefor flattening the DM.

Inter-actuator Stroke ≥ 1.65 µm mechanical for N=64≥ 1.31 µm mechanical for N=84≥ 1.04 µm mechanical for N=112

High order WFE (wavefront error) ≤ 30 nm rms TBC after flattening (refer to thedefinition of “mirror flattening”)

Errors of spatial frequencies greater than thosecorresponding to half the actuator spacing frequency(i.e. errors which can not be self-corrected by theDM).

Surface Roughness ≤ 3 nm (TBC) rms

Scratch/Dig Ratio TBD

Temporal Frequency Response < 5° phase lag at frequencies = or < 500 Hz(TBC) and 10% of max stroke (phase lagdecreasing at lower frequencies)

Hysteresis ≤ 1% of maximum stroke (TBC) For max stroke.

In run actuator repeatability ≤ 6 nm RMS WFE over the entire clear apertureof the mirror when all the actuators arestimulated.

In-run repeatability implies the variation inperformance measured during a single power upusing the same actuator commands (within thedynamic range).

Run-to-run actuator repeatability ≤ 6 nm RMS WFE over the entire clear apertureof the mirror when all the actuators arestimulated.

Run-to-run actuator repeatability is the variation inmeasured performance across a number of devicepower-ups for the same actuator command set.

Reflectivity > 80% from 0.5µm to 0.6µm> 95% from 0.8µm to 1µm> 98% from 1.0µm to 2.5µm

These values should be treated as reflectanceguidelines. The supplier should comment onfeasibility.Durability specifications: any specified minimumreflectance should hold for a minimum TBC 10years in the indicated operational and storageenvironments.

Thermal Radiation When the DM actuators are operated, its opticalsurface temperature will not deviate fromambient temperature by more than 2%.

Laser Applications of AO

Shack-HartmannWavefront Sensor (WFS)

Microlens Array

Wavefront

Detector

Each xy offsetmeasures the local wavefrontslope across thecorrespondinglenslet.

Laser Applications of AO

CurvatureWavefront Sensor

Focal PlaneInput Wavefront

Sensing Planes

Wavefront Sensors andDetectors

• The curvature sensor minimises the number of pixelsrequired to remove a given wavefront variance (assumesKolmogorov or similar)– the use of noiseless fibre-coupled avalanche photo-diodes is

therefore feasible

• Shack-Hartmann requires more pixels so a CCD is normallyemployed– low read-noise multi-port frame-transfer specialised devices– Including on-chip electron-multiplication to effectively

eliminate read noise (multiplication noise effectively decreasesquantum efficiency, however)

• Much of the above is driven by photon starvation in NaturalGuide Star Astro AO– Where there is PLENTY of guide light one can consider other

detectors, especially CMOS.

Detector Quantum Efficiencyfor a CCD with a possiblechoice of dichroic filter

Laser Applications of AO

EEV CCD-39Projected read noise (e- rms) versus pixel

rate/port

Laser Applications of AO

4-port frame transfer CCD

Schematic of

4-port frame-

transfer

CCD

read-out port

shielded frame-

transfer area

light sensitive area

(4 quadrants,

gaps exaggerated)

shift register

(2) charge

movement

(1) frame

transfer

Total pixels

LL CCID-11: 64x64

Loral: 64x64

EEV:80x80

Real-time Computer-RTC (please see poster)

• 1st generation astro AO systems used:– Single PC or RISC device for Real-time (though inevitably

accompanied by other housekeeping processors with typicallyshared memory)

– Multi-CPU– Multi-DSP (most common)

• C40 DSP very popular and still running! • TigerSHARC more recently

• 2nd/3rd generation RTCs incorporate FPGAs for some tasks(and high speed serial comms)

• Cell processor evaluated– Not as good as might be thought for this application

• Future systems (for Extremely Large Telescopes)– Evaluating large FPGAs– And GPUs - very promissing!

Isoplanatic angle, temporalvariation

• Angle over which wavefront distortions are essentially thesame:

83

32 5

2 5 / 3

0

22.91 sec ( )

nC h h dh

!" #$

%& '& '

= ( )( )* +* +,

– It is possible to perform a similar turbulence weighted integral oftransverse wind speed in order to derive an effective wind speedand approximate timescale of seeing

– note the importance of Cn2(h) in both cases.

– LIMITS FIELD OF VIEW OF CONVENTIONAL AO

Correcting two turbulence layers

Deformable mirror

Turbulence Layers

Credit: Rigaut, MCAO for Dummies

Works on axis: bothlayers corrected

Does not work off axis: higher layeruncorrected, lower layer overcorrected

2 Deformable mirrors

Conjugated to eachlayer

Turbulence Layers

Credit: Rigaut, MCAO for Dummies

Multiconjugate AO corrects both layers

Ground Layer AO

• Very large field ofview but only partialcorrection

• Use multiple LGS toisolate the ground-layer turbulence whichapplies to all lines ofsight

• Apply correction withsingle deformablemirror– Often implemented with

an adaptive telescopesecondary mirror

Astronomical AO 27

[Courtesy ESO]

Ground Layer AO with AdaptiveSecondary Deformable Mirrors

28

MMT; being built for LBT, VLT

Compare with“normal” size DM

Laser Tomography AO

• Small field of view andhigh-order correction

• Use multiple LGS toperform tomographyof the turbulentvolume

• Apply correction withsingle deformablemirror

• Overcomes LGS coneeffect

Astronomical AO 29

[Courtesy ESO]Being built for VLT

Multi Object AO• Large field of view and

high-order correction– But individual fields are small

• Use multiple LGS toperform tomography of theturbulent volume

• Then the wavefront can becorrected for eachindividual target direction,by applying correction withmultiple deformablemirrors – one for eachscience target

• Correction is ‘open-loop’ inthat the wavefront is notnulled within a control loop

Astronomical AO 30

[Courtesy ESO]Being studied for 42m E-ELT

Extreme AO (XAO)• Tiny field of view and very

high-order correction• Use single very bright NGS

to analyse wavefront alongsingle line of sight

• Block light from guide starand search for companions

• Apply correction with veryhigh order DM

• Some interesting newtechnologies– Very high order

deformable mirrors (4KMEMS)

– Spatially filtered WFS– Apodised pupil plane Lyot

Coronographs– Auxiliary focal plane and

calibration WFSing

Astronomical AO 31

[OSCA: built UCL, deployed: WHT ]

Specialist Extreme AO planetfinders being built for VLT(SPHERE) and Gemini (GPI)

Calibration interferometer (JPL)measures slow aberrations to

nanometer accuracy

Average IRwavefronts Courtesy: Bruce Macintosh, LLNL