The Challenge of Adaptive Optics

The twinkling of stars is an endearing and romantic characteristic of the night sky. But to professional astronomers wanting the clearest view of faint objects, this celestial shimmer, or seeing, sets a limit on resolution.

How can we beat the seeing without leaving terra firma? Amateur astronomers found a way using video techniques. A video camera (or webcam) takes hundreds of quick exposures to “freeze” an image. By selecting and compositing only the crispest frames, imagers can produce stunning pictures, better than any view through an eyepiece. This technique works best with bright objects that can be recorded with split-second exposures.

Adaptive Optics
Adaptive-optics systems aim to improve the resolution of astronomical observations by compensating for the effects of air turbulence. All techniques have these basic components. The heart of any system is a wave-front detector that can sense how star light is being altered. Then a processor determines how to warp a malleable "rubber" mirror to make split-second corrections. It's like noise-canceling headphones for the stars.
S&T diagram
Professional astronomers, however, must use long exposures with large telescopes to capture the feeble light from the distant universe, and for more than a decade they’ve explored high-tech ways to sharpen the light from stars.

Initial techniques used a wave-front sensor to monitor how a guide star changed and calculated how to flex a deformable mirror to compensate. Govert Schilling's comprehensive article about adaptive optics (AO) appeared in the October 2001 issue of Sky & Telescope.

Early methods were an improvement, but they have problems. By using a laser-generated artificial star or a real star off to the side, the column of air being monitored didn’t exactly match what the main telescope was looking through.

Astronomers hope to improve this by using several reference stars to better characterize the air column over a larger field of view. This multi-conjugate adaptive optics has many designs, but all aim to determine the behavior of different layers of atmosphere (the analysis of these slices of air is called tomography). A Web page at ESO describes many of the various techniques for image correction.

Testing shows promise, but scaling up these advanced AO techniques must leave the drawing boards to prove their legitimacy before the next generation of
extremely large telescopes (ELTs) can meet their potential.