- Laser guide star
Laser guide stars are a form of artificial star created for use in
astronomical adaptive optics imaging.Adaptive optics (AO) systems require awavefront reference source in order to correct atmospheric distortion of light (called "astronomical seeing "). Sufficiently brightstar s are not available in all parts of the sky, which greatly limits the usefulness of naturalguide star adaptive optics. Instead, one can create an artificial guide star by shining alaser into the atmosphere. This star can be positioned anywhere thetelescope desires to point, opening up much greater amounts of the sky to adaptive optics. Because the laser beam is deflected byastronomical 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 atip-tilt mirror . However, this star can be much fainter than is required for natural guide star adaptive optics, which means 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 specially tuned to 589.2
nanometer s to energize a layer ofsodium atoms which is naturally present in themesosphere 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 yellowstreet light s in many cities. Rayleigh beacons rely on the scattering of light by the molecules which make up the loweratmosphere . 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. The lasers are often pulsed, with measurement of the atmosphere being time-gated (taking place a few microseconds after the pulse has been launched so that scattered light at ground level is ignored and only light which has travelled for several microseconds high up into the atmosphere and back is actually detected).Laser guide star adaptive optics is still a very young field, with much effort currently invested in technology development. As of2006 , only two laser guide star AO systems were regularly used for science observations and have contributed to published results inpeer-review ed scientific literature: those at the Lick and Palomar Observatories inCalifornia , and theKeck Observatory inHawaii . However, laser guide star systems were under development at most major telescopes, with theWilliam Herschel Telescope ,Very Large Telescope and Gemini North having tested lasers on the sky but not yet achieved regular operations. Other observatories developing laser AO systems as of 2006 include theLarge Binocular Telescope andGran Telescopio Canarias . The laser guide star system at theVery Large Telescope started regular science operations in June 2007. [ [http://www.eso.org/public/outreach/press-rel/pr-2007/pr-27-07.html Laser Guide Star System on ESO's VLT Starts Regular Science Operations] ]External links
* [http://www.gemini.edu/index.php?option=content&task=view&id=226 Gemini's Laser Vision Reveals Striking New Details in Orion Nebula]
References
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