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ESO tested the new Wendelstein laser guide star unit by shooting a powerful laser beam into the atmosphere. Example of an artificial reference 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. One of the launch telescopes for the VLT Four Laser Guide Star Facility.
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. The sodium atoms then re-emit the laser light, producing a glowing artificial star.