adaptive optics

The term adaptive optics (AO) indicates optical systems that compensate for distortions from the atmosphere using input from a sensor that measures the distortion. In particular, they detect and adjust for changing distortion due to changing air conditions. The technology is used in modern research astronomy and also for laser communications (laser com). Major ground research telescopes (e.g., Keck Observatory) now generally include it.

As used in astronomy, the sensor tracks the distortion (e.g., a slight mislocation) of a known fixed source, such as a star (termed a guide star) or an artificial source of light reflected off the atmosphere itself from a laser (termed an LGS for laser guide star). This fixed source must be near the line of sight to the object under observation. The same distortion presumably affects objects in that region of the celestial sphere (within the field of view), including the observation target. Modern techniques include simultaneous use of multiple guide stars to achieve better performance. Among the advanced techniques are laser tomography AO (LTAO), using multiple LGSs, and multi-conjugate AO (MCAO), typically using multiple LGSs plus a wavefront sensor, modeling sources of distortion at multiple distances from the telescope to better devise the optics adjustments to compensate. Single-conjugate AO (SCAO) indicates use of the wavefront sensor but without modeling more than one source of distortion. The phrase extreme AO (XAO) is used for techniques for better AO than typical; a general strategy is to orient its modeling toward maximizing the benefit to the particular line of sight toward the source-of-interest rather than using a compromise that offers lesser benefits to multiple targets or to the entire field of view. XAO is associated with the use of a coronagraph, e.g., for best possible AO for direct imaging of an extra-solar planet.