Editing Secondary mirror

{{Short description|Element of a reflecting telescope which focuses light gathered by the primary mirror}}
[[File:Secondary Mirror of Keck Telescope.jpg|thumb|right|175px|The secondary mirror of [[W. M. Keck Observatory|Keck Telescope]] in [[Hawaii]].]]
[[File:KeckObservatory20071013.jpg|thumb|right|175px|The secondary mirror assembly of the [[W. M. Keck Observatory|Keck Telescope]] and its relationship to the [[primary mirror]].]]
A '''secondary mirror''' (or '''secondary''') is the second deflecting or focusing mirror element in a [[reflecting telescope]]. Light gathered by the [[primary mirror]] is directed towards a [[Focus (optics)|focal point]] typically past the location of the secondary.<ref>{{Cite web |date=2019-07-04 |title=Observatory: Webb's Mirrors |url=https://jwst.nasa.gov/content/observatory/ote/mirrors/index.html |url-status=live |archive-url=https://web.archive.org/web/20190723142015/https://jwst.nasa.gov/content/observatory/ote/mirrors/index.html |archive-date=2019-07-23 |access-date=2023-10-10 |website=NASA - James Webb Space Telescope - Goddard Space Flight Center |publisher=[[NASA]] |language=en}}</ref><ref>{{Cite web |last=Rocheleau |first=Jake |date=2011-05-10 |title=Secondary Mirror |url=https://planetfacts.org/secondary-mirror/ |url-status=live |archive-url=https://web.archive.org/web/20231103134541/https://planetfacts.org/secondary-mirror/ |archive-date=2023-11-03 |access-date=2023-10-10 |website=Planet Facts |language=en}}</ref> Secondary mirrors in the form of an optically flat ''diagonal mirror'' are used to re-direct the light path in designs such as [[Newtonian reflector]]s. They are also used to re-direct and extend the light path and modify the final image in designs such as [[Cassegrain reflector]]s.<ref>{{Cite web |last=Danek |first=Peter |date=2018-10-28 |title=How To Collimate Mirrors On Newtonian Reflector |url=https://telescopeguides.com/how-to-collimate-mirrors-on-newtonian-reflector/ |url-status=dead |archive-url=https://web.archive.org/web/20221001090439/https://telescopeguides.com/how-to-collimate-mirrors-on-newtonian-reflector/ |archive-date=2022-10-01 |access-date=2023-10-10 |website=Telescope Guides |language=en-US}}</ref>

The secondary is typically suspended by X-shaped struts (sometimes called a "spider") in the path of light between the source and the primary, but can be mounted on other types of mounts or optical elements such as optical windows, or [[Schmidt corrector plate|schmidt]] and [[Meniscus corrector|meniscus]] corrector plates. Employing secondary mirrors in optical systems causes some image distortion due to the obstruction of the secondary itself, and distortion from the spider mounts, commonly seen as cross-shaped [[Diffraction spike|diffraction spikes]] radiating from bright stars seen in astronomical images.

For large telescopes, the secondary mirror can reach a significant size. At the [[Vera C. Rubin Observatory]], the secondary for the Simonyi Survey Telescope is nearly four meters in diameter.<ref>{{cite news
 | title=NSF–DOE Rubin Observatory’s Secondary Mirror Installed
 | date=August 1, 2024 | publisher=NOIRLab
 | url=https://noirlab.edu/public/news/noirlab2419/
 | access-date=2024-10-27
}}</ref>

==See also==
*[[List of telescope parts and construction]]
*[[Mirror mount]]
*[[Mirror support cell]]
*[[Point spread function]]
*[[Primary mirror]]
*[[Silvering]]

== References ==
{{Reflist}}{{DEFAULTSORT:Secondary Mirror}}
[[Category:Mirrors]]
[[Category:Telescopes]]


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