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John O’Byrne School of Physics University of Sydney

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.

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John O’Byrne School of Physics University of Sydney

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

  2. 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? • 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

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

  5. Scidar measurements at SSO 10 minutes of data refractive index structure constant (Cn2 ) v. altitude Adaptive Optics

  6. Seeing parameters - 1 • Fried parameter ro(l,z) = 0.185l6/5cos3/5z(¤Cn2dh)-3/5 • Seeing disk FWHM without AO »l/ro for large telescopes So at ~500nm, ro» 10 cm for 1 arcsec FWHM seeing At 2.5mm, this corresponds to ro» 70 cm and 0.7 arcsec seeing Adaptive Optics

  7. Seeing parameters - 2 If seeing is dominated by a layer at altitude H: • Isoplanatic angle (for wavefront distortion) qo» 0.314 ro/H - typically a few arcsec in visible • Isokinetic angle (for image motion) qk» 0.314 Dtel/H - typically ~100 arcsec in visible • Timescale for wavefront distortion to» 0.314 ro/Vwind - typically ~ few ms • Timescale for image motions tk» 0.314 Dtel/Vwind - typically ~ 100 ms Adaptive Optics

  8. 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

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

  10. WFS - Shearing interferometer The Wavefront Sensor (WFS) may be • Shearing interferometer (uncommon) • Shears the wavefront to measure tilt in the shear direction Adaptive Optics

  11. WFS - 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 Sensor Wavefront Curvature Sensor Uses lenslets to sub divide the aperture and measures curvature of the wavefront in each sub-aperture. Adaptive Optics

  13. 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

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

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

  16. Bimorph mirrors Bimorph mirror made from piezoelectric wafers (sometimes one piezo and one glass) with an electrode pattern to control deformation Adaptive Optics

  17. Membrane Mirrors Continuous faceplates deformed electrostatically by an underlying electrode pattern. Adaptive Optics

  18. Sample of an AO result - 1 Adaptive Optics

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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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

  27. Hokupa’a images - 1 CFHT Adaptive Optics

  28. Hohupa’a Images - 2 QSO PG1700+518 and its companion starbust galaxy. These deep (2hr.) images were made by guiding on the 16th mag QSO itself. Raw AO PSF subtr. Deconlv. J H Adaptive Optics CFHT

  29. Hohupa’a Images - 3 Adaptive Optics GEMINI

  30. 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

  31. Starfire Optical Range (SOR) Adaptive Optics

  32. 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

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