Active Optics - in Astronomy

In Astronomy

Most modern telescopes are reflectors, with the primary element being a very large mirror. Historically, primary mirrors were quite thick in order to maintain the correct surface figure in spite of forces tending to deform it, like wind and the mirror's own weight. This limited their maximum diameter to 5 or 6 metres (200 or 230 inches), such as Palomar Observatory's Hale telescope.

A new generation of telescopes built since the 1980s use thin, lighter weight mirrors instead. They are too thin to maintain themselves rigidly in the correct shape, so an array of actuators is attached to the rear side of the mirror. The actuators apply dynamic forces to the mirror body to keep the reflecting surface in the correct shape. The telescope may also be segmented into multiple smaller mirrors, which can reduce the sagging due to weight that occurs for large, monolithic mirrors.

The combination of actuators, an image quality detector, and a computer to control the actuators to obtain the best possible image, is called active optics.

The name active optics means that the system keeps a mirror (usually the primary) in its optimal shape against environmental forces such as wind, sag, thermal expansion, and telescope axis deformation. Active optics compensate for distorting forces that change relatively slowly, roughly on timescales of seconds. The telescope is therefore actively still, in its optimal shape.