Editing Adaptive optics (section)

== In astronomy ==
=== Atmospheric seeing ===
{{Further|Astronomical seeing}}
[[File:Ao movie.gif|thumb|Negative images of a star through a telescope. The left-hand panel shows the slow-motion movie of a star when the adaptive optics system is switched off. The right-hand panel shows the slow motion movie of the same star when the AO system is switched on. <!--The image of the star is much more compact, and breaks up less with adaptive optics switched on. However, the image of the star also changes shape much more quickly when the adaptive optics system is switched on.-->]]
When light from a star or another astronomical object enters the Earth's atmosphere, atmospheric [[turbulence]] (introduced, for example, by different temperature layers and different wind speeds interacting) can distort and move the image in various ways.<ref>
{{cite conference
|last=Max
|first=Claire
|title=Introduction to Adaptive Optics and its History
|conference=American Astronomical Society 197th Meeting
|url=http://www.cfao.ucolick.org/EO/Resources/History_AO_Max.pdf
}}</ref> Visual images produced by any telescope larger than approximately {{convert|20|cm|m in}} are blurred by these distortions.

=== Wavefront sensing and correction ===
An adaptive optics system tries to correct these [[Optical aberration|distortions]], using a [[wavefront sensor]] which takes some of the astronomical light, a [[deformable mirror]] that lies in the optical path, and a computer that receives input from the detector.<ref>{{cite journal
 | last = Hippler
 | first = Stefan
 | title = Adaptive Optics for Extremely Large Telescopes
 | journal = [[Journal of Astronomical Instrumentation]]
 | volume = 8
 | issue = 2
 | pages = 1950001–322
 | date = 2019
 | doi = 10.1142/S2251171719500016
 | bibcode = 2019JAI.....850001H| arxiv = 1808.02693
 | s2cid = 119505402
 }}</ref> The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a few [[millisecond]]s; the computer calculates the optimal mirror shape to correct the [[Optical aberration|distortions]] and the surface of the [[deformable mirror]] is reshaped accordingly.  For example, an {{convert|8|–|10|m|cm in|adj=on}} telescope (like the [[Very Large Telescope|VLT]] or [[Keck telescope|Keck]]) can produce AO-corrected images with an [[angular resolution]] of 30–60 [[milliarcsecond]] (mas) [[Image resolution|resolution]] at [[infrared]] wavelengths, while the resolution without correction is of the order of 1 [[arcsecond]].

In order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array of [[pixel]]s in a wavefront sensor, either using an array of small [[lenslet]]s (a [[Shack–Hartmann wavefront sensor]]), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixelated map of the wavefronts is fed into the deformable mirror and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of the [[astronomical object]] to be known&nbsp;– even [[Solar System]] objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of the [[Sun]] is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.

=== Using guide stars ===

==== Natural guide stars ====
Because a science target is often too faint to be used as a reference star for measuring the shape of the optical wavefronts, a nearby brighter [[guide star]] can be used instead. The light from the science target has passed through approximately the same atmospheric turbulence as the reference star's light and so its image is also corrected, although generally to a lower accuracy.

The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where a guide star of sufficient [[luminosity]] (for current systems, about [[Apparent magnitude|magnitude]] 12–15) can be found very near to the object of the observation. This severely limits the application of the technique for astronomical observations. Another major limitation is the small field of view over which the adaptive optics correction is good. As the angular distance from the guide star increases, the image quality degrades. A technique known as "multiconjugate adaptive optics" uses several deformable mirrors to achieve a greater field of view.<ref>{{cite journal |last1=Rigaut |first1=François |last2=Neichel |first2=Benoit |title=Multiconjugate Adaptive Optics for Astronomy |journal=Annual Review of Astronomy and Astrophysics |date=14 September 2018 |volume=56 |issue=1 |pages=277–314 |doi=10.1146/annurev-astro-091916-055320|arxiv=2003.03097 |bibcode=2018ARA&A..56..277R }}</ref>

==== 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.}