John O’Byrne School of Physics University of Sydney

John O’Byrne School of Physics University of Sydney Adaptive Optics

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

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. GEMINI http://www.gemini.anu.edu.au/sciops/instruments/adaptiveOptics/Science_drivers.html Adaptive Optics

Where does Seeing arise? Turbulence in the atmosphere leads to refractive index variations. Contributions are concentrated into layers at different altitudes. Adaptive Optics

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 • Vwindis lower (R/Rmax is Strehl resolution normalised by exposure resolution of an infinte aperture) Adaptive Optics

Essentials of an AO system • Wavefront sensor • Computer • Phase modulator Adaptive Optics

Phase 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

Tit-tilt correction Tip-tilt mirror mounted on 4 piezoelectric stacks. Segmented surface deformable mirrors use tip-tilt on individual segments Adaptive Optics

Stacked-array Mirrors Continuous faceplates attached to piezoelectric stacks Visible on the edges of each mirror are the PZT actuators. Adaptive Optics

Sample of an AO result - 1 Adaptive Optics

Sample of an AO result - 2 Core diameter is recovered with low order correction, but a surrounding halo remains Adaptive Optics

AO 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

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

Laser 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

Laser 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

Laser 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

AO 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 / VLThttp://www.eso.org/projects/aot/ Adaptive Optics

AO 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

Hohupa’a Images - 3 Adaptive Optics GEMINI

Keck 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

Starfire Optical Range (SOR) Adaptive Optics

References 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 Adaptive Optics

John O’Byrne School of Physics University of Sydney