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Laser guide star is an artificial star image created for use in astronomical adaptive optics imaging. Adaptive optics (AO) systems require a wavefront reference source in order to correct atmospheric distortion of light (called ''astronomical seeing''). 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. 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. ==Types== There are two main types of laser guide star system, known as sodium and Rayleigh beacon guide stars. Sodium beacons are created by using a laser specially tuned to 589.2 nanometers to energize a layer of sodium atoms that 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 in 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, since the artificial beacon is generated much lower in the atmosphere. The lasers are often pulsed, with measurement of the atmosphere being time-gated (taking place several microseconds after the pulse has been launched, so that scattered light at ground level is ignored and only light that has traveled for several microseconds high up into the atmosphere and back is actually detected). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「laser guide star」の詳細全文を読む スポンサード リンク
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