Adaptive Optics - What is it?

Principle (see right figure)

The aperture size of an optical system puts a limit on the achievable angular resolution. This limit can only be reached if the light wave is not aberrated. For astronomical observations from the ground the light from space has to travel through earth's atmosphere, and will be diffracted at cells of warm and cold air which are mixed by atmospheric turbulence. As a result, the wavefront is aberrated on timescales of tens of milliseconds which is approximately set by the wind speed and the strength of the turbulence. A long exposed image will be blurred and not sharper than that obtained with a small amateur telescope (at least at visible wavelengths).

Shack-Hartmann wavefront sensor


Adaptive Optics flattens the wavefront continuously by measuring the current aberrations with a wavefront sensor and hereafter updating the shape of a deformable mirror (DM) so that the aberration is canceled after reflection. In order to compensate for the atmospheric turbulence, the closed-loop control, which runs on the Control computer, has to run at a framerate of a few hundred Hertz.

The Hartmann method (1900) for testing a lens employs an opaque mask with holes placed behind the lens. Each of the holes acts as an aperture, and as the light is passing through the lens eventually produces an image. The positions of these images resemble local wavefront slopes, which correspond to the holes.

Shack's idea was to use small lenses which are arranged in an array to measure wavefront aberrations. The so called Shack-Hartmann sensor (see left figure above) subdivides the wavefront and images each part separately. As for the Hartmann test, the dislocations of the spot positions represent local wavefront slopes. The control computer estimates the shape of the overall wavefront from these slopes, and derives an appropriate shape of the DM for the correction.

... and the result ...

The movie to the right shows an image sequence of a star observed through the turbulent atmosphere. Some images are open loop, i.e. without any correction. Some are taken with tip-tilt only, i.e. only

Guess which images
belong to which kind of correction.

the image motion is stabilized by a fast steering mirror. Some are closed loop, i.e. image motion is again stabilized, and in addition a deformable mirror flattens the aberrated wavefront.

last update: 4 April 2007
editor of this page: Stefan Hippler