Adaptive Optics 1

John O’ByrneJohn O’Byrne

School of PhysicsSchool of Physics

University of SydneyUniversity of Sydney

Adaptive Optics 2

What is AO?What is AO?

Adaptive OpticsAdaptive Optics: : fast image correction (f fast image correction (f 1 Hz), primarily to correct 1 Hz), primarily to correct

atmospheric wavefront distortionsatmospheric wavefront distortions

Active OpticsActive Optics: : slow image correction (f slow image correction (f 1 Hz), to correct mirror 1 Hz), to correct mirror

and structural deflectionsand structural deflections

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Why do we need AO?Why do we need AO?

ScintillationScintillation - - describes random amplitude fluctuations of describes random amplitude fluctuations of wavefront (twinkling)wavefront (twinkling)

SeeingSeeing - - describes random phase fluctuations of wavefront describes random phase fluctuations of wavefront (image motion and blurring)(image motion and blurring)

AO aims to correct seeing effects - i.e. sharpen imagesAO aims to correct seeing effects - i.e. sharpen images

Science objectivesScience objectives - e.g. GEMINI - e.g. GEMINIhttp://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/Science_drivers.htmlhttp://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/Science_drivers.html

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Where does Seeing arise?Where does Seeing arise?

Turbulence in the atmosphere Turbulence in the atmosphere leads to refractive index variations.leads to refractive index variations.Contributions are concentrated Contributions are concentrated into layers at different altitudes.into layers at different altitudes.

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Scidar measurements at SSOScidar measurements at SSO

10 minutes 10 minutes

of data of data

refractive refractive index index structure structure constant (Cconstant (Cnn

22 ) )

v. altitudev. altitude

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Seeing parameters - 1Seeing parameters - 1

Fried parameter rFried parameter roo(() = 0.185) = 0.1856/56/5coscos3/53/5((CCnn22dhdh))-3/5-3/5

Seeing disk FWHM without AO Seeing disk FWHM without AO /r/ro o for large telescopesfor large telescopes

So at ~500nm, rSo at ~500nm, roo 10 cm 10 cm for for 1 arcsec FWHM seeing 1 arcsec FWHM seeing

At 2.5At 2.5m, this corresponds to rm, this corresponds to roo 70 cm 70 cm and and

0.7 0.7 arcsec seeing arcsec seeing

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Seeing parameters - 2Seeing parameters - 2

If seeing is dominated by a layer at altitude H:If seeing is dominated by a layer at altitude H:

Isoplanatic angle (for wavefront distortion) Isoplanatic angle (for wavefront distortion) oo 0.314 r 0.314 roo/H /H

- typically a few arcsec in visible- typically a few arcsec in visible

Isokinetic angle (for image motion) Isokinetic angle (for image motion) kk 0.314 D 0.314 Dteltel/H /H

- typically ~100 arcsec in visible- typically ~100 arcsec in visible

Timescale for wavefront distortion Timescale for wavefront distortion oo 0.314 r 0.314 roo/V/Vwindwind

- typically ~ few ms- typically ~ few ms

Timescale for image motions Timescale for image motions kk 0.314 D 0.314 Dteltel/V/Vwindwind - -

typically ~ 100 mstypically ~ 100 ms

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What can we expect from AO?What can we expect from AO?

Improvement depends on Improvement depends on

DDteltel relative to r relative to ro o

AO is easier in the infrared AO is easier in the infrared rro o is largeris larger

oo is larger is larger

oo is longer is longer

Also easier if Also easier if H is lowerH is lower VVwindwind is loweris lower

(R/Rmax is Strehl resolution normalised byexposure resolution of an infinte aperture)

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Essentials of an AO systemEssentials of an AO system

Wavefront sensorWavefront sensor ComputerComputer Phase modulatorPhase modulator

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WFS - Shearing interferometerWFS - Shearing interferometer

The Wavefront Sensor (WFS) may beThe Wavefront Sensor (WFS) may be Shearing interferometer (uncommon)Shearing interferometer (uncommon)

Shears the wavefront to measure tilt in the shear Shears the wavefront to measure tilt in the shear directiondirection

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WFS - Shack-Hartmann SensorWFS - Shack-Hartmann Sensor

Shack-Hartmann sensor (the usual choice)Uses lenslets to sub-divide the aperture and measures image motion in each sub-aperture.

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WFS - Curvature SensorWFS - Curvature Sensor

WavefrontWavefront

CurvatureCurvature

Sensor Sensor Uses lenslets to subUses lenslets to sub

divide the aperture anddivide the aperture and

measures curvature ofmeasures curvature of

the wavefront in eachthe wavefront in each

sub-aperture.sub-aperture.

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Phase ModulatorPhase Modulator

The phase modulators are always a deformable mirror The phase modulators are always a deformable mirror

- usually tip-tilt and higher order separately.- usually tip-tilt and higher order separately.

Actuators used:Actuators used: piezoelectric (PZT)piezoelectric (PZT) electrostrictive electrostrictive voice-coil voice-coil electrostaticelectrostatic

But other technologies are possibleBut other technologies are possible Liquid Crystal phase screen devicesLiquid Crystal phase screen devices

More actuators => better correctionMore actuators => better correction ..

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Tit-tilt correctionTit-tilt correction

Tip-tilt mirror mounted on Tip-tilt mirror mounted on

4 piezoelectric stacks.4 piezoelectric stacks.

Segmented surface deformableSegmented surface deformable

mirrors use tip-tilt onmirrors use tip-tilt on

individual segmentsindividual segments

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Stacked-array MirrorsStacked-array Mirrors

Continuous faceplatesContinuous faceplates

attached toattached to

piezoelectric stackspiezoelectric stacks

Visible on the edges of Visible on the edges of

each mirror are the PZTeach mirror are the PZT

actuators.actuators.

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Bimorph mirrorsBimorph mirrors

Bimorph mirror madeBimorph mirror made

from piezoelectric wafersfrom piezoelectric wafers

(sometimes one piezo and(sometimes one piezo and

one glass) with anone glass) with an

electrode pattern to controlelectrode pattern to control

deformationdeformation

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Membrane MirrorsMembrane Mirrors

Continuous faceplatesContinuous faceplates

deformed electrostatically bydeformed electrostatically by

an underlying electrodean underlying electrode

pattern.pattern.

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Sample of an AO result - 1Sample of an AO result - 1

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Sample of an AO result - 2Sample of an AO result - 2

Core diameter is recovered with low order correction, but a surrounding halo remainsCore diameter is recovered with low order correction, but a surrounding halo remains

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AO limitationsAO limitations

AO systems have limitations (e.g. light loss, IR emissivityAO systems have limitations (e.g. light loss, IR emissivity

driven by the large number of optical surfaces) but moredriven by the large number of optical surfaces) but more

fundamental are limits imposed by the guiding star, which isfundamental are limits imposed by the guiding star, which is

monitored by the wavefront sensor, and is likely to bemonitored by the wavefront sensor, and is likely to be

different from the science targetdifferent from the science target

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Natural Guide Stars (NGS)Natural Guide Stars (NGS)

temporal anisoplanatism - delays introduced by the servo temporal anisoplanatism - delays introduced by the servo looploop

angular anisoplanatism - NGS is usually offset from angular anisoplanatism - NGS is usually offset from science target, but can't be too far away or it lies outside science target, but can't be too far away or it lies outside isoplanatic patch angle (isoplanatic patch angle (oo) - can be improved by making ) - can be improved by making

the WFS conjugate to the primary turbulence layer (or the WFS conjugate to the primary turbulence layer (or multiple layers in multi-conjugate AO [MCAO])multiple layers in multi-conjugate AO [MCAO])

WFS sensitivity limit => limited sky coverageWFS sensitivity limit => limited sky coverage

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Laser Guide Stars (LGS) - 1Laser Guide Stars (LGS) - 1

Use a laser to generate a ‘star’ inUse a laser to generate a ‘star’ in

the atmosphere, very close to thethe atmosphere, very close to the

science target’s light path throughscience target’s light path through

the atmosphere. This may be athe atmosphere. This may be a

Rayleigh guide star at 7-20 kmRayleigh guide star at 7-20 km

or a Sodium guide star at 90 km.or a Sodium guide star at 90 km.

Overcomes NGS sky coverage Overcomes NGS sky coverage limitationlimitation

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Laser Guide Stars (LGS) - 2Laser Guide Stars (LGS) - 2

Provides no tip-tilt Provides no tip-tilt informationinformation

Cost!Cost! Problem to other Problem to other

telescopes on the site telescopes on the site caused by back-scattered caused by back-scattered lightlight

Sodium guide star and Rayleigh back-scatterSodium guide star and Rayleigh back-scatter

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Laser Guide Stars (LGS) - 3Laser Guide Stars (LGS) - 3

Focus anisoplanatismFocus anisoplanatism the laser does not fully the laser does not fully

sample the stars light sample the stars light path through the path through the atmosphereatmosphere

worse for a Rayleigh worse for a Rayleigh guide starguide star

provide multiple LGS?provide multiple LGS?

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AO Projects - 1AO Projects - 1AO Projects - 1AO Projects - 1

Australian projectsAustralian projects RSAA 2.3m tip-tilt systemRSAA 2.3m tip-tilt system Anglo-Australian TelescopeAnglo-Australian Telescope

International projectsInternational projects (e.g. see University of Durham list of links to other projects(e.g. see University of Durham list of links to other projects

http://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.htmlhttp://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.html)) GEMINI GEMINI

http://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/AOIndex.htmlhttp://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/AOIndex.html

AO at ESO / VLTAO at ESO / VLT http://www.eso.org/projects/aot/http://www.eso.org/projects/aot/

Adaptive Optics 26

AO Projects - 2AO Projects - 2AO Projects - 2AO Projects - 2

Keck II and now Keck IKeck II and now Keck I http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.htmlhttp://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.html

University of Durham (UK) University of Durham (UK) http://aig-http://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.htmlwww.dur.ac.uk/fix/adaptive-optics/area_main_ao.html

University of HawaiiUniversity of Hawaii most recently Hokupa’a on GEMINI most recently Hokupa’a on GEMINI

http://www.ifa.hawaii.edu/ao/http://www.ifa.hawaii.edu/ao/

Earlier PUEO on CFHT Earlier PUEO on CFHT http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/

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Hokupa’a images - 1Hokupa’a images - 1

CFHTCFHT

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Hohupa’a Images - 2Hohupa’a Images - 2Hohupa’a Images - 2Hohupa’a Images - 2

QSO PG1700+518 and itsQSO PG1700+518 and its

companion starbust galaxy.companion starbust galaxy.

These deep (2hr.) imagesThese deep (2hr.) images

were made by guiding on thewere made by guiding on the

16th mag QSO itself.16th mag QSO itself.

J J H H

Raw AORaw AO

PSF PSF subtr.subtr.

Deconlv.Deconlv.

CFHTCFHT

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Hohupa’a Images - 3Hohupa’a Images - 3Hohupa’a Images - 3Hohupa’a Images - 3

GEMINIGEMINI

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KeckKeck

Keck I AO Keck I AO image in H image in H band taken band taken during the first during the first Keck I AO Keck I AO night night (Dec.12,2000). (Dec.12,2000).

Io angular size: Io angular size: 1.23 arcsecond 1.23 arcsecond Spatial resolution: Spatial resolution: 120 km 120 km

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Starfire Optical Range (SOR)Starfire Optical Range (SOR)

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ReferencesReferences

Information on AO projects can be obtained from their web sites or from theInformation on AO projects can be obtained from their web sites or from the

Proceedings of the (all too frequent) AO conferences (e.g. SPIE, OSA or ESO). Proceedings of the (all too frequent) AO conferences (e.g. SPIE, OSA or ESO).

A few other useful references:A few other useful references:

Popular level:Popular level: Sharper Eyes on the SkySharper Eyes on the Sky - Sky & Space, - Sky & Space, 9,9, 30 (1996) 30 (1996) Untwinkling the Stars - Sky & Telescope, Untwinkling the Stars - Sky & Telescope, 8787, May 24 & Jun 20, (1994), May 24 & Jun 20, (1994) Adaptive Optics - Scientific American, Jun (1994)Adaptive Optics - Scientific American, Jun (1994)

ReviewsReviews:: Young, A.T. (1974), ApJ, Young, A.T. (1974), ApJ, 189189, 587, 587 Roddier, F. (1981), Progress in Optics, Roddier, F. (1981), Progress in Optics, 1919, 281, 281 Coulman ARAA (1985), 23Coulman ARAA (1985), 23, , 1919 Beckers, J.M. (1993), ARAA Beckers, J.M. (1993), ARAA 31,31, 13 13 Wilson, R.W.,Jenkins C.R. (1996), MNRAS, Wilson, R.W.,Jenkins C.R. (1996), MNRAS, 268268, 39, 39