Active optics

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 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.

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,[4] depending on wavelength and weather conditions).

A continuous (non-pulsed) pilot beam can be used to allow for up to 10 kHz bandwidth of stabilization (against vibrations, air turbulence, and acoustic noise) for low repetition rate lasers.

Small changes of the incident wavelength generated by an acousto-optic modulator or interference with a fraction of the incoming radiation delivers the information whether the Fabry Perot is too long or too short.