Used with mirror-based optic systems.
Mirrors are supported on a couple of points. These can be adjusted using actuators.
Due to the light passing through Earths atmosphere, point sources appear to flicker.
At about 93 km above surface is a layer of sodium (Na). In the layer a “star” is created by exciting the atoms of sodium using a laser light source.
These days, 4 fake stars are created. Wavefront detector is used and the data from it fed to control mechanism, which then controls actuators, which in turn shape the mirror to counteract effects of Earths atmosphere.