Laser guide star
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Laser_Guide_Star.jpg
Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging.
Adaptive optics systems require a wavefront reference source in order to correct atmospheric distortion of light (called "astronomical seeing"). Sufficently 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. 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 laser light moves around in the sky in a random fashion. 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.
There are two main types of laser guide star system, known as sodium beacon and Raleigh beacon guide stars. Sodium beacons are created by using a laser specially tuned to 589.2 nanometers to energize a layer of sodium atoms which is naturally present in the mesosphere at an altitude of around 90 kilometers. The sodium atoms then re-emit the laser light, producing a glowing artificial star. The same atomic transition of sodium is used to create bright yellow street lights in many cities. Rayleigh beacons rely on the scattering of light by the molecules that make up the lower atmosphere. In contrast to sodium beacons, Rayleigh beacons are a much simpler and less costly technology, but do not provide as good a wavefront reference as the artificial beacon is generated much lower in the atmosphere.
Laser guide star adaptive optics is still a very young field, with much effort currently being invested in technology development. At the time of this writing in 2004, only one laser guide star AO system, that of the Lick Observatory in California, is regularly used for science observations and has produced published results in the peer-reviewed scientific literature. However, laser guide star systems are under development at most major telescopes, notably including the Keck telescopes, the Very Large Telescope and the William Herschel Telescope.