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{{Short description|Array of smaller mirrors designed to act as one large curved mirror}}
[[File:James Webb Primary Mirror.jpg|thumb|upright=1.25|The first six of 18 mirror segments of the [[James Webb Space Telescope]] are being tested]]
[[File:comparisonComparison optical telescope primary mirrors.svg|thumb|240px280px|Size comparison of primary mirrors. ''Segmented mirrors'' are typically [[hexagonal]] 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 mirror#Mirror shape|spherical]] or asymmetric (if they are part of a larger [[parabolic reflector]]<ref>[httphttps://books.google.com/books?id=9mUL9dGn9V4C&pg=PA107&dq=a+segmented+mirror+segments+of+parabolic+telescope W. Patrick McCray - '''Giant telescopes''' - Page 107]</ref>). They are used as [[Objective (optics)|objectives]] for large [[reflecting telescope]]s. To function, all the mirror segments have to be [[Polishing (metalworking)|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 and necessary technologies were initially developed under the leadership of [[Jerry Nelson (astronomer)|Dr. Jerry Nelson]] at the [[Lawrence Berkeley National Laboratory]] and [[University of California]] during the 1980s, and have since spread worldwide to the point that essentially all future large optical telescopes plan to use segmented mirrors.
 
The concept was pioneered by [[Guido Horn d'Arturo|Guido Horn D'Arturo]], who built the first working segmented mirror in 1952, after twenty years of research;<ref name=":0">[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.]</ref> It was later independently rediscovered and further developed under the leadership of [[Jerry Nelson (astronomer)|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 ==▼
 
▲== Application ==
[[File:Salt mirror.jpg|thumb|left|180px|[[Southern African Large Telescope|SALT]]'s segmented mirror]]▼
[[File:Specchio-tasselli-horn.jpg|left|thumb|Early segmented mirror, built out of 61 hexagonal segments in 1952.<ref name=":0">[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.]</ref>]]
▲[[File:Salt mirror.jpg|thumb|left|180px|[[Southern African Large Telescope|SALT]]'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 mirrormirrors 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 [[telescope]]s.<ref>{{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}}</ref><ref>{{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}}</ref> Using a monolithic mirror much larger than 5&nbsp;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,<ref>{{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 }}</ref><ref>{{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}}</ref> changing the precision shape of the surface. Segments are also easier to fabricate, transport, install, and maintain over very large monolithic mirrors.
| first = Colin
| last = Nickerson
| title = Long time no see
| version =
| publisher = Boston Globe
| date = 2007-11-05
| url = http://www.boston.com/news/globe/health_science/articles/2007/11/05/long_time_no_see/?page=2
| format =
| accessdate = 2009-11-11}}</ref><ref>{{cite web
| title = Keck telescope science kit fact sheet, Part 1
| work =
| publisher = SCI Space Craft International
| date = 2009
| url = http://spacecraftkits.com/KFacts.html
| doi =
| accessdate = 2009-11-11}}</ref> Using a monolithic mirror much larger than 5&nbsp;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,<ref>{{cite journal
| first = Monica Godha
| last = Bobra
| title = The endless mantra: Innovation at the Keck Observatory
| version =
| publisher = MIT
| date = September 2005
| url = http://dspace.mit.edu/bitstream/handle/1721.1/39439/64637845.pdf?sequence=1
| format =
| accessdate = 2009-11-11}}</ref><ref>{{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
| doi =
| accessdate = 2009-11-11}}</ref> 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 [[Smartglasses|smartglass]] element.<ref>{{Cite web|title=tooz technologies|url=https://www.tooztech.com/|access-date=2022-01-26|website=Tooz|language=de}}</ref>
== Telescopes using segmented mirrors==▼
 
▲[[File:comparison optical telescope primary mirrors.svg|thumb|240px|Size comparison of primary mirrors. ''Segmented mirrors'' are typically [[hexagonal]] and arranged in a [[honeycomb]] pattern.]]
 
▲== Telescopes using segmented mirrors==
Some of the [[List of largest optical reflecting telescopes|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|thumb|upright=0.7|The Keck II telescope showing the segmented primary mirror]]
:The twin telescopes[[Keck Telescopes]] are the most prominent of the [[Mauna Kea Observatories]] at an elevation of 4,145 meters (13,600&nbsp;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 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===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.
:The 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&nbsp;m by 9.8&nbsp;m.<ref>{{cite web|title=Mirror Segments|url=http://www.salt.ac.za/telescope/subsystems/mirror/segments/|work=Southern African Large Telescope website|publisher=SOAA|accessdate=10 May 2013}}</ref> It is located close to the town of [[Sutherland, Northern Cape|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]]▼
===Southern African Large Telescope===
:Also known as the GranTeCan, the ''[[Canary Islands|Canaries]] Great Telescope'' uses a total of 36 segmented mirrors.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/6897293.stm ''Tests begin on Canaries telescope''] BBC 14 July 2007</ref><ref>[http://news.yahoo.com/s/ap/20070714/ap_on_sc/giant_telescope Giant telescope begins scouring space] July 14, 2007</ref> With a primary mirror of {{convert|10.4|m|ft|abbr=on}}, it is currently the [[List of largest optical reflecting telescopes|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.▼
▲: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&nbsp;m by 9.8&nbsp;m.<ref>{{cite web |title=Mirror Segments |url=http://www.salt.ac.za/telescope/subsystems/mirror/segments/ |work=Southern African Large Telescope website |publisher=SOAA |accessdateaccess-date=10 May 2013}}</ref> It is located close to the town of [[Sutherland, Northern Cape|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.
* [[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.▼
▲* [[===Gran Telescopio Canarias]]===
* [[James Webb Space Telescope]]▼
▲:Also known as the GranTeCan, the ''[[CanaryGran Telescopio IslandsCanarias|Canaries]] Great Telescope'']] uses a total of 36 segmented mirrors.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/6897293.stm ''Tests begin on Canaries telescope''] BBC 14 July 2007</ref><ref>[httphttps://news.yahoo.com/s/ap/20070714/ap_on_sc/giant_telescope Giant telescope begins scouring space] 14 July 14, 2007</ref> With a primary mirror of {{convert|10.4|m|ft|abbr=on}}, it is currently the [[List of largest optical reflecting telescopes|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.
:The JSWT's 18 mirror segments have mostly been fabricated in 2011.<ref>[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</ref> While its estimated total cost has been rising for several years, it was criticized as ''the telescope that ate astronomy'', because of the ever-increasing fraction of NASA's astrophysics budget it had consumed. {{As of|2013}} the estimated costs amounted to $8.8 billion<ref>http://www.nature.com Lee Billings, [http://www.nature.com/news/2010/101027/full/4671028a.html The telescope that ate astronomy], 27 October 2010</ref> The [[space telescope]] is scheduled to be launched by an [[Ariane 5]] from [[Guiana Space Centre]] in October 2018.
 
▲* [[===LAMOST]]===
▲:The ''[[LAMOST|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.<ref>[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</ref> The [[space telescope]] was launched by an [[Ariane 5]] from [[Guiana Space Centre]] on December 25, 2021.<ref>{{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}}</ref>
 
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== Next -generation telescopes ==
 
{{further|Extremely large telescope}}
 
Three [[extremely large telescope]]s will be the next generation of segmented -mirror telescopes and are planned to be commissioned betweenin 2021 andthe 20242020s. The [[Giant Magellan Telescope]], uses veryseven large segments and is either grouped with segmented mirrors telescopes or its own category. Planned for completion in 2021.<ref>{{Citation |title=All Things Considered |contribution=Mega Mirror To Power Massive New Telescope |contribution-url=http://www.npr.org/2012/01/14/145235055/mega-mirror-to-power-massive-new-telescope |publisher=NPR |date=2012-01-14 |author=Joe Palca |accessdate=2012-03-23}}</ref><ref>http://sen.com, Elizabeth Howell, [http://sen.com/news/giant-telescope-gets-20m-funding-boost-as-design-takes-shape Giant telescope gets $20m funding boost as design takes shape], 29 December 2014</ref> The [[Thirty meterMeter telescopeTelescope]] is beingto be built at the Mauna Kea Observatories in Hawaii, (estimatedthough first-lightconstruction byis 2022)on hold.<ref>http://westhawaiitoday This will use 492 hexagonal segments.com/news/local-news/tmt-begin-construction</ref> The [[European 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 20242027.<ref>http://www.eso.org/{{cite eso1419web — Organisation Release, [http|url=https://www.eso.org/public/newsannouncements/eso1419ann21008/ Groundbreaking|title=ESO's forExtremely theLarge E-ELT],Telescope 19planned to start scientific operations in 2027 |date=11 June 20142021 |access-date=18 June 2021}}</ref>
 
{{multiple image
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|header=Conceptual images of [[Extremely large telescope|ELTELTs]]s
|image1=Latest Rendering of the E-ELT.jpg
|image2=Top view of tmt complex.jpg
|image3=Giant Magellan Telescope - artist's concept.jpg
|caption1=[[European Extremely Large Telescope]]
|caption2=[[Thirty meterMeter telescopeTelescope]]
|caption3=[[Giant Magellan Telescope]]
}}
{{clear}}
 
==Diffraction spikes==
{{excerpt|Diffraction spike|Segmented mirrors}}
 
==See also==
*[[List of largest optical reflecting telescopes]]
▲* [[Hobby-EberlyCherenkov Telescope Array]]
 
==References==
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==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}}
*[http://books.google.com/books?id=rRAiDU3qorkC&pg=PA61&lpg=PA61&dq=diffraction+effects+in+segmented+mirrors&source=bl&ots=Y7wbH4IMkP&sig=s98J5ufF7r0c6m0zBF5-RbsvWkQ&hl=en&ei=rIdTTJWoLcK88gbU87yQBA&sa=X&oi=book_result&ct=result&resnum=8&ved=0CEEQ6AEwBw#v=onepage&q=diffraction%20effects%20in%20segmented%20mirrors&f=false Renaud Foy, Françoise-Claude Foy, Optics in astrophysics]
 
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