Lecture 10: Adaptive Optics (AO): Introduction and Principle Claire Max UC Santa Cruz Wenda Cao Big Bear Solar Observatory New Jersey Institute of Technology Big Bear Solar Observatory Outline AO Introduction Basic Principle Atmospheric Parameters AO’s Eye: Wavefront Sensor AO’s Brain: Control System AO’s Hand: Deformable Mirror AO Application in Solar Observation Textbook: Adaptive Optics Handbook, J M Vaughan http://www.ucolick.org/~max/289/, Dr. Claire Max, UC Santa Cruz Big Bear Solar Observatory 1. AO Introduction Why is adaptive optics needed? Start from a kid’s song … http://www.youtube.com/watch?v=yCjJyiqpAuU “twinkle twinkle little star, how I wonder what you are ……” Turbulence in earth’s atmosphere makes stars twinkle More importantly, turbulence spreads out light; makes it a blob rather than a point Even the largest ground-based telescope have no better resolution than an 8-inch backyard telescope Big Bear Solar Observatory What is Adaptive Optics? Speckles (each is at diffraction limit of telescope) A technique for correcting optical distortions to dramatically improve image quality Useful to astronomy, vision science, laser eye surgery, communication, remote sensing, high-power lasers, …… Big Bear Solar Observatory Light through Turbulence Atmospheric perturbations cause distorted wavefronts Plane Wave Index of refraction variations Distorted Wavefront Big Bear Solar Observatory Light through Turbulence Temperature fluctuations in small patches of air cause changes in index of refraction (like many little lenses) Light rays are refracted many times (by small amounts) When they reach telescope they are no longer parallel Hence rays can’t be focused to a point: Point focus Parallel light rays blur Light rays affected by turbulence Big Bear Solar Observatory Turbulence Turbulence change rapidly with time “Speckle images” movie: sequence of short snapshots of a star, taken at Lick Observatory using an infrared camera Centroid jumps around (image motion) Images spread out into speckles Big Bear Solar Observatory Turbulence Sources stratosphere tropopause 10-12 km wind flow over dome boundary layer ~ 1 km Heat sources w/in dome Big Bear Solar Observatory How does AO Help ? Measure details of blurring from “guide star” near the object you want to observe Calculate (on a computer) the shape to apply to deformable mirror to correct blurring Light from both guide star and astronomical object is reflected from deformable mirror; distortions are removed Big Bear Solar Observatory IR Images of a Star with AO No adaptive optics With adaptive optics Note: “colors” (blue, red, yellow, white) indicate increasing intensity Big Bear Solar Observatory Resolution and Contrast Lick Observatory No AO No AO With AO Intensity With AO Big Bear Solar Observatory Solar Images with AO No adaptive optics With adaptive optics With AO and Speckle Big Bear Solar Observatory Diffraction Limit FWHM ~ /D 1.22 /D With no turbulence, FWHM is diffraction limit of telescope Example: / D = 0.02 arcsec for = 1 m, D = 10 m With turbulence, image size gets much larger (typically 0.5 - 2 arcsec) (rad ) 1.22 in units of /D Point Spread Function (PSF): intensity profile from point source (rad ) Big Bear Solar Observatory D D Turbulence Strength Wavefront of light r0 “Fried’s parameter” Primary mirror of telescope Fried parameter r0 measures the optical quality of the atmosphere r0 is “Coherence Length” corresponding to an area over which the rms wavefront aberration is less that 1 rad r0 indicates the size of a telescope which can just operate at the diffraction limit. r0 ~ 10 - 30 cm at good observing sites Easy to remember: r0 = 10 cm FWHM = 1 arcsec at = 0.5 m Big Bear Solar Observatory Turbulence and Telescope Size For telescope diameter D < r0 , dominant effect is “image wander” , telescope-limit As D >> r0 , seeing-limit For short exposure, many small “speckles” develop. Each speckle is ~ / D For long exposure, these “speckles” are averaged with a overall envelope of ~ / r0 (“seeing disk”) Computer simulations by Nick Kaiser: D=1m D=2m Big Bear Solar Observatory D=8m Turbulence Effect on Image If telescope diameter D >> r0 , image size of a point source is / r0 >> / D /D “seeing disk” / r0 r0 is diameter of the circular pupil for which the diffraction limited image and the seeing limited image have the same angular resolution. r0 10 inches at a good site. So any telescope larger than this has no better spatial resolution! Big Bear Solar Observatory PSF and Strehl Ratio Intensity Definition of “Strehl”: Ratio of peak intensity to that of “perfect” optical system x AO produces point spread function with a “core” and “halo” When AO system performs well, more energy in core When AO system is stressed (poor seeing), halo contains larger fraction of energy (diameter ~ r0) Ratio between core and halo varies during night Big Bear Solar Observatory 2. AO Principle Feedback loop: next cycle corrects the (small) errors of the last cycle Big Bear Solar Observatory Wavefront Sensor (WFS) Shack-Hartmann wavefront sensor: one method among many to measure turbulent distortions Big Bear Solar Observatory Shack-Hartmann WFS f y f Big Bear Solar Observatory Shack-Hartmann WFS Shack-Hartmann wfs measures local “tilt” of wavefront Divide pupil into subapertures of size ~ r0 – Number of subapertures (D / r0)2 Lenslet in each subaperture focuses incoming light to a spot on the wavefront sensor’s CCD detector Deviation of spot position from a perfectly square grid measures shape of incoming wavefront Wavefront reconstructor computer uses positions of spots to calculate voltages to send to deformable mirror Big Bear Solar Observatory Control System High Speed Baja Camera Sensor: Wavelength: Format: Frequency: Frame Rate: ROI: Camera Output: Readout: Transportation: Electronics: PB-MV13 CMOS Visible 1280 1024 pixel 66 MHz 2500 Hz over 200 200 20 20 pixel 10 bit Parallel Ripple 10 ports via Camera Link Extension to DSP FPGA Big Bear Solar Observatory Control System 40 DSPs ch0 ADSP-21160 SHARC DSP ch1 SMART Baja AO76 ch2 INTERFACE ch3 CAMERA ch4 Camera To DSPs 200x200 ch5 10 ports Sorts ch6 Pixels ch7 Into Subapertures 66 MHz ch8 Link Port To RS422 Deformable Mirror Tip/Tilt Mirror D/A Monitor 2500 fps ch9 Keyboard Host Computer/AO Control System Big Bear Solar Observatory Motor Controller WFS field Stop motor Deformable Mirror (DM) BEFORE Incoming Wave with Aberration AFTER Deformable Mirror Corrected Wavefront Big Bear Solar Observatory DM for Real Wavefronts Big Bear Solar Observatory Deformable Mirror In practice, a small deformable mirror with a thin bendable face sheet is used Real deformable mirrors (DM) have smooth surfaces Placed after the main telescope mirror Most deformable mirrors today have thin glass face-sheets Big Bear Solar Observatory Deformable Mirror Structure Glass face-sheet Light Cables leading to mirror’s power supply (where voltage is applied) PZT or PMN actuators: get longer and shorter as voltage is changed Anti-reflection coating Big Bear Solar Observatory Deformable Mirror Type Deformable mirrors come in many sizes Range from 13 to > 900 actuators (degrees of freedom) Xintics is a leader in the development of deployment of DMs About 12” A couple of inches Xinetics Big Bear Solar Observatory Tiny Deformable Mirror Potential for less cost per degree of freedom Liquid crystal devices – Voltage applied to back of each pixel changes index of refraction locally (not ready for prime time yet) MEMS devices (micro-electro-mechanical systems) - very promising today Electrostatically Membrane actuated Attachment mirror diaphragm post Continuous mirror Big Bear Solar Observatory Laser Guide Star Laser guide stars are used for night-time telescopes If there is no close-by “real” star, create one with a laser Use a laser beam to create artificial “star” at altitude of 100 km in atmosphere Big Bear Solar Observatory Galactic Center with Keck LGS Keck laser guide star AO Best natural guide star AO Big Bear Solar Observatory Astronomical AO: World Tour Big Bear Solar Observatory Astronomical AO: World Tour Hawaii Big Bear Solar Observatory Summit of Mauna Kea in Hawaii Subaru 2 Kecks Gemini North Big Bear Solar Observatory AOs on Large Telescopes Keck Observatory, Hawaii 2 10-m telescopes European Southern Observatory, Chile 4 telescopes Gemini North Telescope, Hawaii Subaru Telescope, Hawaii MMT Telescope, Arizona New Solar Telescope, Big Bear, CA Soon: Gemini South Telescope, Chile Large Binocular Telescope, Arizona Advanced Technology Solar Telescope, Hawaii Big Bear Solar Observatory Adaptive Optic Position • Example: AO system at Lick Observatory’s 3 m telescope Support for main telescope mirror Adaptive optics package below main mirror Big Bear Solar Observatory DM Wavefront sensor Off-axis parabola mirror IRCAL infra-red camera Big Bear Solar Observatory Adaptive Optic Position • Example: Palomar AO system AO system is in Cassegrain cage 200” Hale telescope Big Bear Solar Observatory Adaptive Optic Position • The Keck Telescopes Adaptive optics lives here Big Bear Solar Observatory Nasmyth platform Person! Big Bear Solar Observatory 3. New Discovery with AO Two images from Palomar of a brown dwarf companion to GL 105 200” telescope No AO With AO Credit: David Golimowski Big Bear Solar Observatory New Discovery with AO 2.3 arcsec Neptune in infra-red light (1.65 microns) Without AO With Keck AO Big Bear Solar Observatory New Discovery with AO Neptune at 1.6 m: Keck AO exceeds resolution of Hubble Space Telescope HST NICMOS Keck AO 2.3 arcsec 2.4 meter telescope 10 meter telescope Big Beardates Solar and Observatory (Two different times) New Discovery with AO Uranus with Hubble Space Telescope and Keck AO L. Sromovsky HST, Visible Keck AO, IR Lesson: Keck in near IR has ~ same resolution as Hubble in visible Big Bear Solar Observatory New Discovery with AO Uranus with Hubble Space Telescope and Keck AO de Pater HST, Visible Keck AO, IR Big Bear Solar Observatory Open Questions for AO Current systems (natural and laser guide stars): How can we measure the Point Spread Function while we observe? How accurate can we make our photometry? astrometry? What methods will allow us to do high-precision spectroscopy? Future systems: Can we push new AO systems to achieve very high contrast ratios, to detect planets around nearby stars? How can we achieve a wider AO field of view? How can we do AO for visible light (replace Hubble on the ground)? How can we do laser guide star AO on future 30-m telescopes? Big Bear Solar Observatory Key Technology in AO New kinds of deformable mirrors with > 5000 degrees of freedom Wavefront sensors that can deal with this many degrees of freedom Innovative control algorithms “Tomographic wavefront reconstuction” using multiple laser guide stars New approaches to doing visible-light AO Big Bear Solar Observatory 5. AO Applications Astronomy Biology Imaging the living human retina Improving performance of microscopy (e.g. of cells) Free-space laser communications (thru air) Imaging and remote sensing (thru air) Big Bear Solar Observatory Imaging Human Retina Why is AO needed for imaging the living human retina? Around edges of lens and cornea, imperfections cause distortion In bright light, pupil is much smaller than size of lens, so distortions don’t matter much But when pupil is large, incoming light passes through the distorted regions Edge of lens Pupil Results: Poorer night vision (flares, halos around streetlights). Can’t image the retina very clearly (for medical applications) Big Bear Solar Observatory Eye PSF and Pupil Size 1 mm 2 mm 3 mm 4 mm 5 mm Perfect Eye 6 mm 7 mm AO Typical Eye Big Bear Solar Observatory C. of Austin Roorda AO for Astronomy Sky LGS wavefront sensor laser beacon imaging Big Bear Solar Observatory wavefront corrector AO for Astronomy Sky LGS wavefront sensor laser beacon imaging Big Bear Solar Observatory wavefront corrector Eye AO for Vision Science illumination wavefront sensor laser beacon imaging Big Bear Solar Observatory wavefront corrector Eye AO for Vision Science illumination wavefront sensor laser beacon imaging Big Bear Solar Observatory wavefront corrector AO for Vision Science Eye wavefront sensing laser beacon vision testing Big Bear Solar Observatory wavefront correction Imaging Human Retina Austin Roorda, UC Berkeley Without AO With AO: Resolve individual cones (retina cells that detect color) Big Bear Solar Observatory Horizontal Path Application Horizontal path thru air: r0 is tiny! So-called “strong turbulence” regime 1 km propagation distance, typical daytime turbulence: r0 can easily be only 1 or 2 cm Turbulence produces “scintillation” (intensity variations) in addition to phase variations Isoplanatic angle also very small Angle over which turbulence correction is valid 0 ~ r0 / L ~ (1 cm / 1 km) ~ 2 arc seconds (10 rad) Big Bear Solar Observatory Laser Communications 10’s to 100’s of gigabits/sec Example: AOptix Applications: flexibility, mobility HDTV broadcasting of sports events Military tactical communications Between ships, on land, land to air Big Bear Solar Observatory
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