SCAO: Single Conjugate Adaptive Optics
If the astronomers know a true star (a natural guide star) that is close to (or: in the) observation area, then this star can be used for derforming the mirror.
Different types of wavefront sensors are in use: In Shack–Hartmann SCAO systems the wavefront sensor is an array of lenslets that measures the average gradient (slopes) of the phase over each subaperture in the pupil plane.
|Schematic operation of a Shack Hartmann sensor. In the ideal case, the lenslets deliver a periodic image of the guide star (top). Under a perturbed wavefront, this image bcomes irregular (bottom). Source: http://www.ctio.noao.edu/~atokovin/tutorial/part3/wfs.html|
MCAO and MOAO: Increasing the angle to be viewed.
The disadvantage of SCAO systems is the narrow field of view (typically less than 1 arc minute). Multi conjugate adaptive optic systems (MCAO) and multi object adaptive optics systems (MOAO)use artificial laser guide stars or combinations of laser guide stars and natural guidestars to increase the angle of view to several arc minutes. Such laser guide stars are obtained by sendig laser beams (like in Star Wars) into the sky which are then reflected at the sodium layer that surrounds the earth at a height of about 90 km. Due to the finite distance of this sodium layer, techniques from tomography have to be applied to detect the atmospheric turbulence at different heights of the atmosphere.
A merry Christmas to all of you.