Editing Laser guide star (section)

==Types==

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| image1            = The first 22-watt sodium laser of the Adaptive Optics Facility.jpg
| image2            = One of the launch telescopes for the VLT Four Laser Guide Star Facility.jpg
| caption1          = The first 22-watt [[TOPTICA]] sodium laser of the Adaptive Optics Facility<ref>{{cite news|title=Powerful New Laser Passes Key Test|url=http://www.eso.org/public/announcements/ann14020/|accessdate=2 April 2014|newspaper=ESO}}</ref>
| caption2          = One of the launch telescopes for the VLT Four Laser Guide Star Facility.<ref>{{cite news|title=VLT's New Laser Launchers Arrive at ESO|url=http://www.eso.org/public/announcements/ann12012/|accessdate=22 February 2012|newspaper=ESO Announcement}}</ref>
}}

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 [[nanometer]]s to energize atoms in the [[sodium layer]] of the [[mesosphere]] at an altitude of around {{convert|90|km|abbr=on}}. 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 lamp]]s for [[street light]]ing.

Rayleigh beacons rely on the [[Rayleigh scattering|scattering]] of light by the molecules in the lower atmosphere. In contrast to sodium beacons, Rayleigh beacons are much simpler and less costly, 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).