Editing Adaptive optics (section)

==== Artificial guide stars ====
[[Image:Laser Towards Milky Ways Centre.jpg|thumb|A laser beam directed toward the centre of the [[Milky Way]]. This laser beam can then be used as a guide star for the AO.]]
An alternative is the use of a [[laser beam]] to generate a reference light source (a [[laser guide star]], LGS) in the atmosphere. There are two kinds of LGSs: [[Rayleigh scattering|Rayleigh]] guide stars and [[sodium]] guide stars. Rayleigh guide stars work by propagating a [[laser]], usually at near [[ultraviolet]] wavelengths, and detecting the backscatter from air at altitudes between {{cvt|15|and|25|km|ft}}. Sodium guide stars use laser light at 589 [[nanometer|nm]] to resonantly excite sodium atoms higher in the [[mesosphere]] and [[thermosphere]], which then appear to "glow". The LGS can then be used as a wavefront [[reference]] in the same way as a natural guide star&nbsp;– except that (much fainter) natural reference stars are still required for image position (tip/tilt) information. The [[lasers]] are often pulsed, with measurement of the [[atmosphere]] being limited to a window occurring a few [[microsecond]]s after the pulse has been launched. This allows the system to ignore most scattered light at ground level; only light which has travelled for several microseconds high up into the atmosphere and back is actually detected.}