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((short description|Main light-gathering source of reflecting telescope))
((More citations needed|date=November 2023))
[[File:James Webb Primary Mirror.jpg|thumb|right|Six of the 18 primary mirrors of the [[James Webb Space Telescope]] being prepared for [[acceptance testing]].]]
[[File:A20010288000 NASM2017-10014 (cropped).jpg|thumb|right|The correctly ground backup primary mirror built by Eastman Kodak for the Hubble space telescope (the mirror was never coated with a reflective surface, hence its [[Honeycomb mirror|honeycomb]] support structure is visible). It now resides in the [[National Air and Space Museum]] in Washington, DC.<ref>((cite web |date=2007-08-28 |title=Mirror, Primary Backup, Hubble Space Telescope |url=http://collections.nasm.si.edu/code/emuseum.asp?profile=objects&newstyle=single&quicksearch=A20010288000 |url-status=dead |archive-url=https://web.archive.org/web/20071013111541/http://collections.nasm.si.edu/code/emuseum.asp?profile=objects&newstyle=single&quicksearch=A20010288000 |archive-date=2007-10-13 |access-date=2008-04-26 |website=Smithsonian - National Air and Space Museum |language=en))</ref>]]
[[File:LBT 3.JPG|thumb|right|The largest non-segmented mirror in an optical telescope in 2009, one of the Large Binocular Telescope's two mirrors.]]

A '''primary mirror''' (or '''primary''') is the principal [[etendue|light-gathering]] surface (the [[objective (optics)|objective]]) of a [[reflecting telescope]].

==Description==
The primary mirror of a reflecting telescope is a [[Curved mirrors|spherical]] or [[Parabolic reflector|parabolic]] shaped disks of polished reflective metal ([[speculum metal]] up to the mid 19th century), or in later telescopes, glass or other material coated with a reflective layer. One of the first known reflecting telescopes, [[Newton's reflector|Newton's reflector of 1668]], used a 3.3&nbsp;cm polished metal primary mirror. The next major change was to use silver on glass rather than metal, in the 19th century such was with the [[Crossley reflector]]. This was changed to vacuum deposited aluminum on glass, used on the 200-inch Hale telescope.

Solid primary mirrors have to sustain their own weight and not deform under gravity, which limits the maximum size for a single piece primary mirror.

[[Segmented mirror]] configurations are used to get around the size limitation on single primary mirrors. For example, the [[Giant Magellan Telescope]] will have seven 8.4 meter primary mirrors, with the resolving power equivalent to a ((convert|24.5|m|ft|1|abbr=on)) optical aperture.<ref>((cite magazine |title=Giant telescope in race to become world's largest |date=2007-10-04 |author=Maggie McKee |magazine=[[New Scientist]] |url=https://www.newscientist.com/article/dn12742-giant-telescope-in-race-to-become-worlds-largest.html |access-date=2009-03-27))</ref>

==Superlative primary mirrors==

The largest optical telescope in the world as of 2009 to use a non-segmented single-mirror as its primary mirror is the ((convert|8.2|m|ft|abbr=on)) [[Subaru (telescope)|Subaru telescope]] of the [[National Astronomical Observatory of Japan]], located in [[Mauna Kea Observatory]] on [[Hawaii (island)|Hawaii]] since 1997;<ref>((cite web |date=2007-05-21 |title=Hi-tech - The Subaru Telescope |url=https://web-japan.org/kidsweb/hitech/subaru/index.html |url-status=live |archive-url=https://web.archive.org/web/20230410123643/https://web-japan.org/kidsweb/hitech/subaru/index.html |archive-date=2023-04-10 |access-date=2024-01-22 |website=Kids Web Japan |language=en))</ref>((Better source needed|reason=The current source is insufficiently reliable ([[WP:NOTRS]]).|date=January 2024)) however, this is not the largest diameter single mirror in a telescope, the U.S./German/Italian [[Large Binocular Telescope]] has two ((convert|8.4|m|ft|abbr=on)) mirrors (which can be used together for interferometric mode).<ref name="BBC">((cite web |date=2008-03-06 |title=Giant telescope opens both eyes |url=http://news.bbc.co.uk/2/hi/science/nature/7282385.stm |url-status=live |archive-url=https://web.archive.org/web/20230710062427/http://news.bbc.co.uk/2/hi/science/nature/7282385.stm |archive-date=2023-07-10 |access-date=2008-03-06 |work=BBC News |language=en))</ref> Both of these are smaller than the 10 m segmented primary mirrors on the [[W. M. Keck Observatory|dual Keck telescope]]. The [[Hubble Space Telescope]] has a ((convert|2.4|m|ftin|abbr=off)) primary mirror.

Radio and submillimeter telescopes use much larger dishes or antennae, which do not have to be made as precisely as the mirrors used in optical telescopes. The [[Arecibo Telescope]] used a 305 m dish, which was the world largest single-dish radio telescope fixed to the ground.<ref>((cite web |date=2004-12-01 |title=The 305 meter radio telescope |url=http://www.naic.edu/public/the_telescope.htm |url-status=dead |archive-url=https://web.archive.org/web/20150204114533/http://www.naic.edu/public/the_telescope.htm |archive-date=2015-02-04 |access-date=2015-01-22 |website=National Astronomy and Ionosphere Center - Arecibo Observatory))</ref><ref name="guardian collapsed">((cite news |date=2020-12-01 |title=Giant Arecibo radio telescope collapses in Puerto Rico |url=https://www.theguardian.com/world/2020/dec/01/arecibo-radio-telescope-collapses-puerto-rico |url-status=live |archive-url=https://web.archive.org/web/20230325192711/https://www.theguardian.com/world/2020/dec/01/arecibo-radio-telescope-collapses-puerto-rico |archive-date=2023-03-25 |access-date=2020-12-01 |work=[[The Guardian]] |language=en |agency=[[Associated Press]]))</ref> The [[Green Bank Telescope]] has the world's largest steerable single radio dish with 100 m in diameter.<ref>((cite web |date=2011-05-20 |title=Green Bank |url=https://science.nrao.edu/about/greenbank |url-status=live |archive-url=https://web.archive.org/web/20230813151230/https://science.nrao.edu/about/greenbank |archive-date=2023-08-13 |access-date=2015-01-22 |website=National Radio Astronomy Observatory |language=en))</ref> There are larger radio arrays, composed of multiple dishes which have better [[image resolution]] but less [[Sensitivity (electronics)|sensitivity]].<ref>((cite web |date=2005-02-22 |title=Radio Telescope |url=http://abyss.uoregon.edu/~js/glossary/radio_telescope.html |url-status=dead |archive-url=https://web.archive.org/web/20221006013247/http://abyss.uoregon.edu/~js/glossary/radio_telescope.html |archive-date=2022-10-06 |access-date=2015-01-22 |website=University of Oregon |language=en-US))</ref>

==See also==
* [[Active optics]]
* [[Honeycomb mirror]]
* [[Liquid-mirror telescope]]
* [[List of largest optical reflecting telescopes]]
* [[List of telescope parts and construction]]
* [[Mirror mount]]
* [[Mirror support cell]]
* [[Secondary mirror]]
* [[Silvering]]

==References==
((reflist))

[[Category:Optical telescope components]]
[[Category:Mirrors]]

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