Segmented mirror#Application
{{Short description|Array of smaller mirrors designed to act as one large curved mirror}}
File:Comparison optical telescope primary mirrors.svg and arranged in a honeycomb pattern.]]
A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector[https://books.google.com/books?id=9mUL9dGn9V4C&pg=PA107&dq=a+segmented+mirror+segments+of+parabolic+telescope W. Patrick McCray - Giant telescopes - Page 107]). They are used as objectives for large reflecting telescopes. To function, all the mirror segments have to be polished to a precise shape and actively aligned by a computer-controlled active optics system using actuators built into the mirror support cell.
The concept was pioneered by Guido Horn D'Arturo, who built the first working segmented mirror in 1952, after twenty years of research;[https://articles.adsabs.harvard.edu/pdf/2018MmSAI..89..448B Bonoli, F. "Guido Horn d'Arturo and the first multi-mirror telescopes: 1932-1952." Memorie della Societa Astronomica Italiana 89 (2018): 448.] It was later independently rediscovered and further developed under the leadership of Dr. Jerry Nelson at the Lawrence Berkeley National Laboratory and University of California during the 1980s{{Citation needed span|text=|date=December 2021|reason=See discussion.}}, and since then all the necessary technologies have spread worldwide to the point that essentially all future large optical telescopes plan to use segmented mirrors.
Application
File:Specchio-tasselli-horn.jpg
File:Salt mirror.jpg's segmented mirror]]
There is a technological limit for primary mirrors made of a single rigid piece of glass. Such non-segmented, or monolithic mirrors can not be constructed larger than about eight meters in diameter. The largest monolithic mirrors in use are currently the two primary mirrors of the Large Binocular Telescope, each with a diameter of 8.4 meters. The use of segmented mirrors is therefore a key component for large-aperture telescopes.{{cite news |first=Colin |last=Nickerson |title=Long time no see |newspaper=Boston Globe |date=5 November 2007 |url=http://www.boston.com/news/globe/health_science/articles/2007/11/05/long_time_no_see/?page=2 |access-date=11 November 2009}}{{cite web |title=Keck telescope science kit fact sheet, Part 1 |publisher=SCI Space Craft International |date=2009 |url=http://spacecraftkits.com/KFacts.html |access-date=11 November 2009}} Using a monolithic mirror much larger than 5 meters is prohibitively expensive due to the cost of both the mirror, and the massive structure needed to support it. A mirror beyond that size would also sag slightly under its own weight as the telescope was rotated to different positions,{{cite journal |first=Monica Godha |last=Bobra |title=The endless mantra: Innovation at the Keck Observatory |publisher=MIT |date=September 2005 |url=http://dspace.mit.edu/bitstream/handle/1721.1/39439/64637845.pdf?sequence=1 |access-date=11 November 2009 |url-status=dead |archive-url=https://web.archive.org/web/20110605000037/http://dspace.mit.edu/bitstream/handle/1721.1/39439/64637845.pdf?sequence=1 |archive-date=5 June 2011 }}{{cite web |last=Yarris |first=Lynn |title=Revolution in telescope design debuts at Keck after birth here |work=Science@Berkeley Lab |publisher=Lawrence Berkeley Laboratory |date=Winter 1992 |url=http://www.lbl.gov/Science-Articles/Archive/keck-telescope.html |access-date=11 November 2009}} changing the precision shape of the surface. Segments are also easier to fabricate, transport, install, and maintain over very large monolithic mirrors.
Segmented mirrors do have the drawback that each segment may require some precise asymmetrical shape, and rely on a complicated computer-controlled mounting system. All of the segments also cause diffraction effects in the final image.
Another application for segmented mirrors can be found in the augmented reality sector to minimize the size of the optical components. A partial reflective segmented mirror array is used by [https://tooz.com/ tooz] to out-couple the light from their light guides, which is used as an optical smartglass element.{{Cite web|title=tooz technologies|url=https://www.tooztech.com/|access-date=2022-01-26|website=Tooz|language=de}}
Telescopes using segmented mirrors
Some of the largest optical telescopes in the world use segmented primary mirrors. These include, but are not limited to the following telescopes:
=Keck Telescopes=
Image:KeckObservatory20071013.jpg
The twin Keck Telescopes are the most prominent of the Mauna Kea Observatories at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in Hawaii, United States. Both telescopes feature {{convert|10|m|ft|abbr=on}} primary mirrors.
=Hobby-Eberly Telescope=
The Hobby-Eberly Telescope (HET) is a 9.2-meter (30-foot) telescope located at the McDonald Observatory, West Texas at an altitude of 2,026 m (6,647 ft). Its primary mirror is constructed from 91 hexagonal segments. The telescope's main mirror is fixed at a 55 degree angle and can rotate around its base. A target is tracked by moving the instruments at the focus of the telescope; this allows access to about 70–81% of the sky at its location and a single target can be tracked for up to two hours.
=Southern African Large Telescope=
The Southern African Large Telescope (SALT) is a 10-meter telescope dedicated on spectroscopy for most of its observing time. It shares similarities with the Hobby-Eberly Telescope and also consists of 91 hexagonal mirror segments, each 1 meter across, resulting in a total hexagonal mirror of 11.1 m by 9.8 m.{{cite web |title=Mirror Segments |url=http://www.salt.ac.za/telescope/subsystems/mirror/segments/ |work=Southern African Large Telescope website |publisher=SOAA |access-date=10 May 2013}} It is located close to the town of Sutherland in the semi-desert region of the Karoo, South Africa. It is a facility of the South African Astronomical Observatory, the national optical observatory of South Africa.
=Gran Telescopio Canarias=
Also known as the GranTeCan, the Canaries Great Telescope uses a total of 36 segmented mirrors.[http://news.bbc.co.uk/1/hi/sci/tech/6897293.stm Tests begin on Canaries telescope] BBC 14 July 2007[https://news.yahoo.com/s/ap/20070714/ap_on_sc/giant_telescope Giant telescope begins scouring space] 14 July 2007 With a primary mirror of {{convert|10.4|m|ft|abbr=on}}, it is currently the world's largest optical telescope, located at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands in Spain.
=LAMOST=
The Large Sky Area Multi-Object Fibre Spectroscopic Telescope is a survey telescope located in the Hebei Province of China. It consists of two rectangular mirrors, made up of 24 and 37 segments, respectively. Each hexagonal segment is 1.1 metre in size.
=James Webb Space Telescope=
The 18 mirror segments of the James Webb Space Telescope were mostly fabricated in 2011.[http://news.softpedia.com/news/Six-JWST-Mirror-Segments-to-Begin-Cryogenic-Tests-195206.shtml "Six JWST Mirror Segments to Begin Cryogenic Tests"] news.softpedia.com 14 April 2011 The space telescope was launched by an Ariane 5 from Guiana Space Centre on December 25, 2021.{{Cite web|last=Margetta|first=Robert|date=2021-09-08|title=NASA Readies James Webb Space Telescope for December Launch|url=http://www.nasa.gov/press-release/nasa-readies-james-webb-space-telescope-for-december-launch|access-date=2021-09-15|website=NASA}}
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| caption1=Mirror segments made of beryllium
| caption2=Cryogenic tests at the MSFC
| caption3=Segment after being coated with gold
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Next-generation telescopes
{{further|Extremely large telescope}}
Three extremely large telescopes will be the next generation of segmented-mirror telescopes and are planned to be commissioned in the 2020s. The Giant Magellan Telescope uses seven large segments and is either grouped with segmented mirrors telescopes or its own category. The Thirty Meter Telescope is to be built at the Mauna Kea Observatories in Hawaii, though construction is on hold. This will use 492 hexagonal segments. The Extremely Large Telescope will be the largest of all three, using a total of 798 segments for its primary mirror. Its first light is expected for 2028.{{Cite web |last=information@eso.org |title=Timeline {{!}} ELT {{!}} ESO |url=https://elt.eso.org/about/timeline/ |access-date=2024-12-19 |website=elt.eso.org |language=en}}{{Cite web |author1=Samantha Mathewson |date=2024-12-09 |title=A 'giant' rising in the desert: World's largest telescope comes together (photo) |url=https://www.space.com/the-universe/a-giant-rising-in-the-desert-worlds-largest-telescope-comes-together-photo |access-date=2024-12-19 |website=Space.com |language=en}}
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| image1 = Latest Rendering of the E-ELT.jpg
| image2 = Top view of tmt complex.jpg
| image3 = Giant Magellan Telescope - artist's concept.jpg
| caption1 = Extremely Large Telescope
| caption2 = Thirty Meter Telescope
| caption3 = Giant Magellan Telescope
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Diffraction spikes
{{excerpt|Diffraction spike|Segmented mirrors}}
See also
References
{{Reflist|30em}}
External links
- {{cite book |chapter-url=https://books.google.com/books?id=rRAiDU3qorkC&pg=PA61 |chapter=Segmented Mirror Telescopes |title=Optics in Astrophysics |publisher=Springer Science+Business Media |first=Jerry |last=Nelson |editor1-first=Renaud |editor1-last=Foy |editor2-first=François-Claude |editor2-last=Foy |pages=61–72 |date=2005 |isbn=978-1-4020-3437-4}}
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