Editing Laser guide star (section)

{{Short description|Artificial star image used by telescopes}}[[File:The most powerful laser guide star system in the world sees first light at the Paranal Observatory.jpg|thumb|Powerful laser guide star system at the [[Paranal Observatory]].]] [[File:Photograph of a Laser guide star.png|thumb|The actual laser guide star is the small spot above the apparent end of the laser beam.]]A '''laser guide star''' is an artificial [[star]] image created for use in [[astronomical]] [[adaptive optics]] systems, which are employed in large [[telescope]]s in order to correct [[atmospheric]] distortion of light (called ''[[astronomical seeing]]'').  Adaptive optics (AO) systems require a [[wavefront]] reference source of light called a [[guide star]].  Natural stars can serve as point sources for this purpose, but sufficiently bright stars are not available in all parts of the sky, which greatly limits the usefulness of natural guide star adaptive [[optics]]. Instead, one can create an artificial guide star by shining a [[laser]] into the [[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.