Adaptive optics technology requires three components: a wavefront sensor, which detects the atmospheric distortions and blurring in light coming from space; a computer to receive and process this input; and a deformable mirror, controlled by the computer, that corrects the distortions. An adaptive optics system must update the mirror's shape several hundred times per second.
The most basic adaptive optics sytems uses a bright star as a reference beacon; the light from this star is used as the wavefront sensor's input. This imposes a significant limitation on the use of this technology: it is only useful for observing objects that happen to be within a few arcseconds of a star bright enough to be a reference beacon.
A possible solution is to use multiple guide stars to examine the tomography (sections) of the atmospheric distortion of the sky. This method allows astronomers to calculate the aberration in the section of sky of the object they are observing.
A more sophisticated adaptive optics technology provides its own reference data using a laser. For example, the Mount Palomar Observatory's laser guided optics system uses two polarized lasers with wavelengths of 589 nm (yellow) and 660 nm (red). The lasers' energy excites sodium atoms in a particular part of the atmosphere (the "sodium layer"). The sodium atoms give off light that is detected by the wavefront sensor, allowing the adaptive optics sytem to be used on any part of the sky.
