. 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 uses 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 variable forces to the mirror body to keep the reflecting surface in the correct shape over repositioning. The telescope may also be segmented into multiple smaller mirrors, which 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.
Comparison with adaptive optics Active optics should not be confused with
adaptive optics, which operates on a much shorter timescale to compensate for atmospheric effects, rather than for mirror deformation. The influences that active optics compensate (temperature, gravity) are intrinsically slower (1 Hz) and have a larger amplitude in aberration. Adaptive optics on the other hand corrects for
atmospheric distortions that affect the image at 100–1000 Hz (the
Greenwood frequency, mirror in a telescope. == Other applications ==