active optics

A method of maintaining a highly accurate optical surface in a reflecting telescope by means of a computer-controlled feedback system that continually monitors the quality of the image and uses the information to adjust a motorized support system under the mirror. The development of this technique means that mirrors can be made from thinner, less massive blanks that can be supported in a lighter structure. If the mirror is flexible enough, problems such as spherical aberration and astigmatism can be greatly reduced by applying constant forces at a large number of positions (perhaps fifty or more) on the back of the mirror. Variations in the forces on the mirror, such as the flexure under gravity as the telescope moves, can be corrected at intervals as short as a few minutes. The term has also been used to describe the technique now more commonly called adaptive optics.