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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
Adaptive Optics 3
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
Adaptive Optics 4
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.
Adaptive Optics 5
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
Adaptive Optics 6
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
Adaptive Optics 7
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
Adaptive Optics 8
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)
Adaptive Optics 9
Essentials of an AO systemEssentials of an AO system
Wavefront sensorWavefront sensor ComputerComputer Phase modulatorPhase modulator
Adaptive Optics 10
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
Adaptive Optics 11
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.
Adaptive Optics 12
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.
Adaptive Optics 13
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 ..
Adaptive Optics 14
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
Adaptive Optics 15
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.
Adaptive Optics 16
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
Adaptive Optics 17
Membrane MirrorsMembrane Mirrors
Continuous faceplatesContinuous faceplates
deformed electrostatically bydeformed electrostatically by
an underlying electrodean underlying electrode
pattern.pattern.
Adaptive Optics 19
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
Adaptive Optics 20
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
Adaptive Optics 21
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
Adaptive Optics 22
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
Adaptive Optics 23
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
Adaptive Optics 24
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?
Adaptive Optics 25
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/
Adaptive Optics 28
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
Adaptive Optics 29
Hohupa’a Images - 3Hohupa’a Images - 3Hohupa’a Images - 3Hohupa’a Images - 3
GEMINIGEMINI
Adaptive Optics 30
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
Adaptive Optics 32
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
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