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John O’Byrne School of Physics University of Sydney. What is AO?. Adaptive Optics : fast image correction (f ³ 1 Hz), primarily to correct atmospheric wavefront distortions Active Optics : slow image correction (f £ 1 Hz), to correct mirror and structural deflections. - PowerPoint PPT Presentation
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Adaptive Optics 1
John O’Byrne
School of Physics
University of Sydney
Adaptive Optics 2
What is AO?What is AO?
Adaptive Optics: fast image correction (f ³ 1 Hz), primarily to correct
atmospheric wavefront distortions
Active Optics: slow image correction (f £ 1 Hz), to correct mirror and
structural deflections
Adaptive Optics 3
Why do we need AO?Why do we need AO?
Scintillation - describes random amplitude fluctuations of wavefront (twinkling)
Seeing - describes random phase fluctuations of wavefront (image motion and blurring)
AO aims to correct seeing effects - i.e. sharpen images
Science objectives - e.g. GEMINIhttp://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 leads to refractive index variations.Contributions are concentrated into layers at different altitudes.
Adaptive Optics 5
What can we expect from AO?What can we expect from AO?
Improvement depends on
Dtel relative to ro
AO is easier in the infrared ro is larger qo is larger to is longer
Also easier if H is lower Vwind is lower(R/Rmax is Strehl resolution normalised by
exposure resolution of an infinte aperture)
Adaptive Optics 6
Essentials of an AO systemEssentials of an AO system
Wavefront sensor Computer Phase modulator
Adaptive Optics 7
Phase ModulatorPhase Modulator
The phase modulators are always a deformable mirror
- usually tip-tilt and higher order separately.
Actuators used: piezoelectric (PZT) electrostrictive voice-coil electrostatic
But other technologies are possible Liquid Crystal phase screen devices
More actuators => better correction.
Adaptive Optics 8
Tit-tilt correctionTit-tilt correction
Tip-tilt mirror mounted on
4 piezoelectric stacks.
Segmented surface deformable
mirrors use tip-tilt on
individual segments
Adaptive Optics 9
Stacked-array MirrorsStacked-array Mirrors
Continuous faceplates
attached to
piezoelectric stacks
Visible on the edges of
each mirror are the PZT
actuators.
Adaptive Optics 10
Sample of an AO result - 1Sample of an AO result - 1
Adaptive Optics 11
Sample of an AO result - 2Sample of an AO result - 2
Core diameter is recovered with low order correction, but a surrounding halo remains
Adaptive Optics 12
AO limitationsAO limitations
AO systems have limitations (e.g. light loss, IR emissivity
driven by the large number of optical surfaces) but more
fundamental are limits imposed by the guiding star, which is
monitored by the wavefront sensor, and is likely to be
different from the science target
Adaptive Optics 13
Natural Guide Stars (NGS)Natural Guide Stars (NGS)
temporal anisoplanatism - delays introduced by the servo loop
angular anisoplanatism - NGS is usually offset from science target, but can't be too far away or it lies outside isoplanatic patch angle (qo) - can be improved by making the WFS conjugate to the primary turbulence layer (or multiple layers in multi-conjugate AO [MCAO])
WFS sensitivity limit => limited sky coverage
Adaptive Optics 14
Laser Guide Stars (LGS) - 1Laser Guide Stars (LGS) - 1
Use a laser to generate a ‘star’ in
the atmosphere, very close to the
science target’s light path through
the atmosphere. This may be a
Rayleigh guide star at 7-20 km
or a Sodium guide star at 90 km.
Overcomes NGS sky coverage limitation
Adaptive Optics 15
Laser Guide Stars (LGS) - 2Laser Guide Stars (LGS) - 2
Provides no tip-tilt information
Cost! Problem to other
telescopes on the site caused by back-scattered light
Sodium guide star and Rayleigh back-scatter
Adaptive Optics 16
Laser Guide Stars (LGS) - 3Laser Guide Stars (LGS) - 3
Focus anisoplanatism the laser does not fully
sample the stars light path through the atmosphere
worse for a Rayleigh guide star
provide multiple LGS?
Adaptive Optics 17
AO Projects - 1AO Projects - 1
Australian projects RSAA 2.3m tip-tilt system Anglo-Australian Telescope
International 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.html) GEMINI
http://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/AOIndex.html
AO at ESO / VLT http://www.eso.org/projects/aot/
Adaptive Optics 18
AO Projects - 2AO Projects - 2
Keck II and now Keck I http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.html
University of Durham (UK) http://aig-www.dur.ac.uk/fix/adaptive-optics/area_main_ao.html
University of Hawaii most recently Hokupa’a on GEMINI
http://www.ifa.hawaii.edu/ao/
Earlier PUEO on CFHT http://www.cfht.hawaii.edu/Instruments/Imaging/AOB/
Adaptive Optics 19
Hohupa’a Images - 3Hohupa’a Images - 3
GEMINI
Adaptive Optics 20
KeckKeck
Keck I AO image in H band taken during the first Keck I AO night (Dec.12,2000).
Io angular size: 1.23 arcsecond Spatial resolution: 120 km
Adaptive Optics 21
Starfire Optical Range (SOR)Starfire Optical Range (SOR)
Adaptive Optics 22
ReferencesReferences
Information 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).
A few other useful references:
Popular level: Sharper Eyes on the Sky - Sky & Space, 9, 30 (1996) Untwinkling the Stars - Sky & Telescope, 87, May 24 & Jun 20, (1994) Adaptive Optics - Scientific American, Jun (1994)
Reviews: Young, A.T. (1974), ApJ, 189, 587 Roddier, F. (1981), Progress in Optics, 19, 281 Coulman ARAA (1985), 23, 19 Beckers, J.M. (1993), ARAA 31, 13 Wilson, R.W.,Jenkins C.R. (1996), MNRAS, 268, 39