How Does an Adaptive Optics System Work?

There are several AO systems working in both astronomy and vision science, but all work in closely similar fashions.

Consider a beam of parallel light passing through a vacuum; a slice across this beam will contain some pattern of phases which will move (uniformly) at the speed of light along the beam. If the beam passes through a uniform medium, its speed is slowed but the pattern of phases still moves together. In a non-uniform medium, however, some parts of the beam are slowed more than others, leading to distortions in the uniform wavefront.

All AO systems work by determining the shape of the distorted wavefront, and using an "adaptive" optical element -- usually a deformable mirror -- to restore the uniform wavefront by applying an opposite cancelling distortion.

The most basic systems use a point source of light as a reference beacon, whose light is used to probe the shape of the wavefronts. This may be a bright star, or in the case of vision research a laser spot focused on the retina. Light from this reference source is analysed by a wavefront sensor, and then commands are sent to actuators (pistons) which change the surface of a deformable mirror to provide the necessary compensations. For the system to work well, it must respond to wavefront changes while they are still small; for the earth's atmosphere, this means updating the mirror's shape several hundred times a second!

Click here for a movie illustrating the results of Adaptive Optics (MPEG1, 1.8MB).

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