primary mirror
{{short description|Main light-gathering source of reflecting telescope}}
{{More citations needed|date=November 2023}}
File:James Webb Primary Mirror.jpg being prepared for acceptance testing.]]
File:A20010288000 NASM2017-10014 (cropped).jpg support structure is visible). It now resides in the National Air and Space Museum in Washington, DC.{{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}}]]
A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope.
Description
The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic 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 of 1668, used a 3.3 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.{{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}}
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 of the National Astronomical Observatory of Japan, located in Mauna Kea Observatory on Hawaii since 1997;{{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}}{{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).{{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}} Both of these are smaller than the 10 m segmented primary mirrors on the 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.{{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}}{{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}} The Green Bank Telescope has the world's largest steerable single radio dish with 100 m in diameter.{{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}} There are larger radio arrays, composed of multiple dishes which have better image resolution but less sensitivity.{{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}}