A Short Adaptive Optics Tutorial

Twinkling stars result from high altitude atmospheric turbulence. This ever present curse of high resolution astronomy has only recently been removed by the application of real-time, high-speed, active optical systems, called adaptive optics.

In the past, only expensive military programs or some of the world's larger observatories used adaptive optics systems. Using experience gained in some of these programs, Stellar Products developed the AO-2.

Atmospheric turbulence can be described by the seeing quality; better seeing means that the atmosphere is more uniform over your telescope aperture. A parameter called the coherence diameter essentially describes the same thing. If the coherence diameter is larger than your telescope diameter, then the seeing is excellent. If the coherence diameter is smaller than your telescope diameter, the seeing is terrible. Mauna Kea, one of the world's best observing sites, usually has a coherence diameter between 4" and 12". This means that even medium size telescopes hardly ever perform to their best optical diffraction limit.

The most complex and expensive adaptive optic systems correct many of the distortions produced by the atmosphere. Theoretically, the distorted light wavefront coming from a single star is approximated by a series of different aberrations, including tip, tilt, defocus, coma, astigmatism, spherical, and other complex surfaces. The lowest order aberrations, tip and tilt, are responsible for most of the distortions caused by the atmosphere. Telescope vibration, whether caused by wind or ground motion, is also composed of only tip and tilt. For these reasons, the AO-2 only corrects these aberrations.

Removing the tip and tilt aberrations actually triples the wavefront quality. For example, for an 8" telescope, the atmospheric wavefront quality for a coherence diameter of 8" will improve from about l/2 down to l/6! Many telescopes are built to the Rayleigh criterion of l/4 optics, considered the minimum acceptable quality level. If the atmosphere is only l/2, then the total system performance will be degraded to an unacceptable level. The value of the AO-2 is to effectively remove the atmosphere!

When viewing extended objects like planets, it is likely that each direction looks through a different atmospheric distortion. The angle over which the distortion is the same is called the isoplanatic patch. Typically, this patch diameter is only a few arcseconds, making complex adaptive optics useless for all but single stars or the very smallest targets. The isoplanatic patch for just tip and tilt, however, is much larger. This is because the lower order aberrations are due to larger patches of wind in the upper atmosphere. Fortunately, this diameter is just about the size of Jupiter or Venus, making the AO-2 tip/tilt adaptive optics very effective for planetary photography!

For the same reason, the tip and tilt frequencies are also much lower than for higher order aberrations. Tip and tilt aberrations at 30 Hz, for example, are usually already below the optical diffraction limit, so no correction is required at that frequency. The AO-2 can remove all tip and tilt errors down to the diffraction limit. Brighter targets have better performance, but even small telescopes work well on the brighter planets.

For additional information on adaptive optics and image stabilization, the following articles are suggested reading.

"Concerning the Problem of Making Sharper Photographs of the Planets", R. Leighton, Scientific American, Vol 194, June 1956, p. 157. This paper presented to the Amateur Scientist editor the "clearest color photographs of Saturn and Mars" and Jupiter, using a low frequency image stabilizer on a stopped-down Mt. Wilson telescope. (The color magazine paper originals show much better than microfilm or xerographic copies.)

"An Image Stabilization Experiment at the Canada-France-Hawaii Telescope", R. Racine and R. McClure, Publications of the Astronomical Society of the Pacific, Vol 101, #642, Aug 1989, p. 731. This article includes photographs showing improved resolution due to an image stabilization system on Mauna Kea on a 3.6 m aperture telescope.

"Untwinkling the Stars" Part I in Sky & Telescope, p.24, May, 1994, and Part II on p. 20, June 1994. These articles show the future of laser guide stars in adaptive optics, and explain how astronomers use adaptive optics.

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