Laser guide star

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.

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 tuned to 589.2 nanometers to energize atoms in the sodium layer of the mesosphere at an altitude of around 90 km (56 mi). The sodium atoms then re-emit the laser light, producing a glowing artificial star. The same atomic transition of sodium is used in sodium-vapor lamps for street lighting.

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