Editing Adaptive optics (section)

==== 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>