An Introduction to Adaptive Optics Mike Hein PH 464 – Applied Optics Winter 2005

Why do we need adaptive optics?  To Correct for Irregularities in the Transmission Media –Atmospheric (Astronomical)  Scintillation or Twinkling (Eddies ~ √λL)  Beam Wander or Quiver (Eddies > Beam Size)  Spreading (Eddies < Beam Size) –Atmospheric (LASER Transmission)  Thermal Blooming –Ocular (Retinal Imaging)  Defects in the Lens and Cornea

How Turbulence Effects Light Spherical waves emitted by the source (star) are essentially plane waves when they arrive at the Earth’s atmosphere. The waves are then randomly distorted by turbulence in the atmosphere.

Checking the Figure of the Hale Telescope

How does it work?  Two Basic Functions: –Wavefront sensing  Shack-Hartmann wavefront sensor  Shearing interferometer –Wavefront correction  Tip-tilt optical element  Deformable optical element

Wavefront Sensing  Shack-Hartmann Sensor –Has become the standard type of sensor –Uses an array of spherical “lenselets” –Focuses wavefronts onto a CCD array –(b) is a plane wave –(c) is a distorted wave

Wavefront Sensing  Shearing Interferometer –Distorted wavefronts result in phase shifted output signals between the detector arrays. –Two interferometers are required for correction in the x-y plane.

Wavefront Correction  High speed tip-tilt mirror reduces overall wavefront tilt  Deformable mirror (monolithic or segmented) corrects wavefront shape

Wavefront Correction  Monolithic Deformable Mirror –Reflective face cast on a solid piezoelectric backing (1.4um). –Thin face supported by an array of electro-mechanical actuators (+/- 3.0um) –Thin face supported by an array of piezoelectric actuators (5.0um)

Wavefront Correction  Segmented Mirrors –Capable of larger corrections than monolithic types. –Requires frequent calibration –Diffraction due to gaps between segments

Some Definitions Angular isoplanatism – the angular measure over which a compensated wavefront can be considered planar. At visible wavelengths the isoplanatic angle is about 2 arc seconds increasing to 10 arc seconds in near infrared light. Fried Parameter (r 0 ) – the turbulence coherence length. A measure of the size of turbulent cells in the atmosphere. Typically 5cm to 20cm.

Some Definitions  Apparent or Visual Magnitude – The relative brightness of a star as seen by an observer on Earth  Guide Star – A sufficiently bright star to use as a reference for image compensation. The required magnitude varies as a function of the desired observation wavelength

Limitations of Image Compensation  Compensation in visible wavelengths (0.5um) requires a guide star of magnitude 10 or brighter.  Compensation in infrared wavelengths (2.2um) allows guide stars down to magnitude 14.  Small isoplanatic region around the guide star limits total observation area: –Visual – 1/100,000 of the sky –IR – 1/1000 of the sky

Guide Stars

Artificial Guide Stars  The area of sky coverage can be expanded using artificial guide stars.  Created using a LASER aligned with the telescope optics.  Rayleigh beacons take advantage of Rayleigh scattering in the atmosphere. –Useful for telescopes with apertures < 2m. –Creates a 1 to 2 arc second guide star 5 to 10km in altitude. –100W LASER –LASER is pulsed so that backscatter can be eliminated by range gating.

Artificial Guide Stars  Sodium Beacons exploit a layer of sodium vapor in the upper atmosphere –10W LASER at 589nm –Excites sodium atoms 90 to 92 km in altitude.  Photo - The Sodium laser launching from the side of the Lick Observatory 120" Shane Telescope. This is a 10 minute time exposure - note the star trails

Artificial Guide Stars  Natural guide star required for tilt correction.  Limiting magnitude for guide star is about 20.  Result is nearly 100% sky coverage.

IR Stellar imaging from Lick Observatory

 Upper Left: Io image taken with Keck adaptive optics; K- band, 2.2 micron.  Upper Right: Io image based on visible light taken with Galileo spacecraft orbiter.  Lower Left: Io image taken with Keck adaptive optics; L-band, 3.5micron.  Lower Right: Io image taken without Keck adaptive optics.

The Lick Observatory CIS

Non-Astronomical Applications  Most of the current AO technology was developed by DARPA during the Cold War to image and shoot down satellites with LASER weapons.

Retinal Imaging

References  Carroll, Joseph, Daniel C. Gray, Austin Roorda, David R. Williams, “Recent Advances in Retinal Imaging with Adaptive Optics,” Optics and Photonics News, Jan 2005: p  Chaisson, Eric and Steve McMillan, Astronomy Today 4 th Edition, Prentice Hall, Upper Saddle River, NJ, 2002  Florence, Ronald, The Perfect Machine, Harper Collins, New York, NY, 1994  Hardy, John W., Adaptive Optics for Astronomical Telescopes, Oxford University Press, New York, NY, 1998  Hardy, John W., “Adaptive Optics,” Scientific American, June 1994: p  Hecht, Eugene, Optics 4 th Edition, Addison-Wesley, Reading, MA, 2002  Tyson, Robert K., Principles of Adaptive Optics 2 nd Edition, Academic Press, San Diego, CA, 1998  AdaptiveOptics.com, 2002, Adaptive Optics Associates Inc.,  Center for Adaptive Optics, 2005, University of California Santa Cruz,