Laser guide star

A laser guide star is an artificial star image created for use in astronomicaladaptive optics systems, which are employed in large telescopes in order to correct atmospheric distortion of light, a phenomenon termed astronomical seeing. Adaptive optics (AO) systems require a wavefront reference source of light called a guide star. While natural stars can serve as point sources for this purpose, sufficiently bright stars are not common enough to appear in all parts of the sky, limiting the usefulness of adaptive optics in these areas. To compensate for this, an artificial guide star can be created by shining a laser into Earth's atmosphere. Light from the beam is reflected by components in the upper atmosphere back into the telescope. This star can be positioned anywhere the telescope desires to point, opening up much greater amounts of the sky to adaptive optics.

Because the laser beam is deflected by astronomical seeing on the way up, the returning laser light does not move around in the sky as astronomical sources do. In order to keep astronomical images steady, a natural star nearby in the sky must be monitored in order that the motion of the laser guide star can be subtracted using a tip-tilt mirror. However, this star can be much fainter than is required for natural guide star adaptive optics because it is used to measure only tip and tilt, and all higher-order distortions are measured with the laser guide star. This means that many more stars are suitable, and a correspondingly larger fraction of the sky is accessible.