View source for Extremely large telescope

{{Short description|20-100-m-aperture astronomical observatory}}
{{for|the specific observatory by this name|Extremely Large Telescope}}
{{comparison optical telescope primary mirrors.svg|upright{{=}}2|the above extremely large telescopes and}}

An '''extremely large telescope''' ('''ELT''') is an [[astronomical observatory]] featuring an [[optical telescope]] with an [[aperture]] for its [[primary mirror]] from 20 metres up to 100 metres across,<ref>As  [http://www.astro-opticon.org/fp5/skelcase.html A Skeleton Science Case For Extremely Large (20m–100m) Ground-based Telescopes (ELTs)] and first section of [https://www.science.org.au/sites/default/files/user-content/resources/file/eltroadmap040917.pdf ELT Roadmap] {{webarchive|url=https://web.archive.org/web/20150518234711/https://www.science.org.au/sites/default/files/user-content/resources/file/eltroadmap040917.pdf |date=2015-05-18 }}, PDF</ref><ref name="NYT-20240308">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=Good News and Bad News for Astronomers’ Biggest Dream - The National Science Foundation takes a step (just one) toward an “extremely large telescope.” |url=https://www.nytimes.com/2024/03/08/science/astronomy-extremely-large-telescope.html |date=8 March 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240308112703/https://www.nytimes.com/2024/03/08/science/astronomy-extremely-large-telescope.html |archivedate=8 March 2024 |accessdate=8 March 2024 }}</ref> when discussing [[reflecting telescope]]s of optical wavelengths including [[ultraviolet]] (UV), [[visible light|visible]], and [[infrared|near infrared]] wavelengths. Among many planned capabilities, extremely large telescopes are planned to increase the chance of finding Earth-like [[Exoplanet|planets around other stars]].<ref>{{Cite news |url=https://www.theguardian.com/science/2006/aug/05/spaceexploration.universe |title=Extremely Large Telescope could reveal secrets of life, the universe and everything |work=[[The Guardian]] |first=Alok |last=Jha |date=5 August 2006}}</ref> Telescopes for radio wavelengths can be much bigger physically, such as the {{convert|300|m|yd|abbr=off}} aperture fixed focus radio telescope of the [[Arecibo Observatory]] (now defunct). Freely steerable radio telescopes with diameters up to {{convert|100|m|yd|abbr=off}} have been in operation since the 1970s.

These telescopes have a number of features in common, in particular the use of a [[segmented mirror|segmented primary mirror]] (similar to the existing [[Keck telescopes]]), and the use of high-order [[adaptive optics]] systems.<ref name=constr>{{Cite journal |url=http://www.tmt.org/news/TMT-Construction%20Proposal-Public.pdf |publisher=TMT Observatory Corporation |page=29 |title=Thirty Meter Telescope Construction Proposal |date=2007-09-12 |access-date=2009-07-24 |journal= |archive-date=2016-03-24 |archive-url=https://web.archive.org/web/20160324230243/http://tmt.org/news/TMT-Construction%20Proposal-Public.pdf |url-status=dead }}</ref><ref name=design_optics>{{Cite book |title=GMT Conceptual Design Report |chapter=Chapter 6: Optics |url=http://www.gmto.org/CoDRpublic |chapter-url=http://www.gmto.org/codrfolder/GMT-ID-01467-Chapter_6_Optics.pdf |pages=6–3 |access-date=2008-04-02 |publisher=GMT Consortium}}</ref>

Although extremely large telescope designs are large, they can have smaller apertures than the [[aperture synthesis]] on many large optical [[Interferometry|interferometers]]. However, they may collect much more light, along with other advantages.

==List of telescopes==
{| class="wikitable sortable" style="text-align: center; font-size:100%;"
! #
! class="unsortable" | Image
! Name
! data-sort-type="number" | Aperture (m)
! Area (m<sup>2</sup>)
! Primary mirror
! Altitude (m)
! First<br />light
! class="unsortable" | Notes
! class="unsortable" | Refs
|-
| 1
| align=left nowrap | [[File:Latest Rendering of the E-ELT.jpg|100px]] 
| [[European Extremely Large Telescope|Extremely Large Telescope]]<br />(ELT)
| 39.3
| 978
| 798 × 1.45&nbsp;m<br />hexagonal {{f/}}1
| 3060
| 2028
| Under construction at [[Cerro Armazones Observatory|Cerro Armazones Obs.]], Chile
|<ref name="eso-groundbreaking2014">{{cite web |url=http://www.eso.org/public/news/eso1419/ |title=Groundbreaking for the E-ELT |date=19 June 2014}}</ref><ref name=govert>{{cite news |url=https://www.science.org/content/article/europe-downscales-monster-telescope-save-money |first=Govert |last=Schilling |title=Europe Downscales Monster Telescope to Save Money |date=14 June 2011 |work=Science Insider}}</ref><ref>{{Cite web |url=http://www.eso.org/public/astronomy/teles-instr/e-elt_num.html |title=The E-ELT in numbers |access-date=2009-08-11 |archive-url=https://web.archive.org/web/20090821225427/http://www.eso.org/public/astronomy/teles-instr/e-elt_num.html |archive-date=2009-08-21 |url-status=dead }}</ref>
|-
| 2
| align=left nowrap | [[File:Top view of tmt complex.jpg|100px]] 
|[[Thirty Meter Telescope]]<br />(TMT)
| 30.0
| 655
| 492 × 1.45&nbsp;m<br />hexagonal {{f/}}1
| 4050
| 2030s
| Construction approved at [[Mauna Kea Observatory|Mauna Kea Obs.]] in Hawaii, USA, halted as of September 2019 due to protests
|<ref name=constr /><ref name=operations>{{Citation |url=http://www.tmt.org/observatory/timeline |title=Thirty Meter Telescope timeline page |publisher=TMT Observatory Project |access-date=2010-10-12 |archive-date=2010-09-25 |archive-url=https://web.archive.org/web/20100925013506/http://www.tmt.org/observatory/timeline |url-status=dead }}</ref><ref>[https://www.tmt.org/page/timeline?category=Observatory+Construction TMT Timeline], accessed February 11, 2018</ref><ref>{{cite news |url=https://www.bbc.com/news/world-us-canada-46046864 |title=Hawaii top court approves controversial Thirty Meter Telescope |date=31 October 2018 |work=BBC News}}</ref>
|-
| 3
| align=left nowrap | [[File:Giant Magellan Telescope - artist's concept.jpg|100px]] 
|[[Giant Magellan Telescope]]<br />(GMT)
| 24.5
| 368
| 7 × 8.4&nbsp;m<br />circular {{f/}}0.71
| 2516
| 2029
| Under construction at [[Las Campanas Observatory|Las Campanas Obs.]], Chile;<br />6/7 mirrors cast
|<ref name=design_optics /><ref>{{cite news |first=Elizabeth |last=Howell |url=http://sen.com/news/giant-telescope-gets-20m-funding-boost-as-design-takes-shape |title=Giant telescope gets $20m funding boost as design takes shape |date=29 December 2014 |work=Sen |access-date=8 January 2015 |archive-date=17 July 2021 |archive-url=https://web.archive.org/web/20210717164947/https://sen.com/news/giant-telescope-gets-20m-funding-boost-as-design-takes-shape |url-status=dead }}</ref><ref>{{Cite web |date=12 March 2021 |title=Giant Magellan Telescope – 6th mirror cast |url=https://www.universetoday.com/150506/the-giant-magellan-telescopes-6th-mirror-has-just-been-cast-one-more-to-go/ |access-date=11 July 2023}}</ref>
|- style="background: #FFFFE0;" 
| 4
| align=left nowrap | [[File:LBT 2.png|100px]]
| [[Large Binocular Telescope]]<br />(LBT)
| 11.8 <br />(''equiv. area'')<br />22.8 <br />(''equiv. [[diffraction|detail]] limit'')
| 111
| 2 × 8.4&nbsp;m<br />circular
| 3221
| 2008
| Largest non-segmented mirrors.<br />Located on [[Mount Graham]], Arizona, USA
|<ref>{{cite press release |title=Large Binocular Telescope Achieves First Binocular Light |url=http://medusa.as.arizona.edu/lbto/firstbinocularlight_press_release.htm |date=2008-02-28 |publisher=Large Binocular Telescope Corporation |url-status=dead |archive-url=https://web.archive.org/web/20080310010642/http://medusa.as.arizona.edu/lbto/firstbinocularlight_press_release.htm |archive-date=2008-03-10}}</ref>
|-
! colspan=10 style="font-size: 0.9em; font-weight: normal; text-align: left; padding: 4px 2px 6px 4px;" |''Note:'' Aperture of LBT: the baseline is obtained via [[aperture synthesis]].
|}

<gallery class="center" mode="packed" heights="180px">
File:Size comparison between the E-ELT and other telescope domes.jpg|The domes of the [[Extremely Large Telescope|ELT]], the [[Thirty Meter Telescope|TMT]] and the [[Giant Magellan Telescope|GMT]] compared to other well-known telescopes
</gallery>

==Budget==
Possible budget figures, which are estimates and can vary over time. For construction costs, it is recommended to estimate the cost of a giant telescope with the following equation:<ref>{{Cite web|url=http://www.gsmt.noao.edu/documentation/SPIE_Papers/Stepp.pdf|title=Estimating the costs of extremely large telescopes |first1=Larry |last1=Stepp |first2=Larry |last2=Daggert |first3=Paul |last3=Gillett |website=National Optical Astronomy Observatory}}</ref>

<math>cost \varpropto D^{2.7}</math>
{| class="wikitable sortable" style="text-align:left;font-size:100%;"
! Name
! Cost<br />(est. USD)
! Alternate
|-
| [[European Extremely Large Telescope|Extremely Large Telescope]] (ELT)
| $1590 million
| €1300 million<ref>{{Cite web |title=Funding boost for ESO’s Extremely Large Telescope |url=https://www.eso.org/public/announcements/ann20034/ |access-date=29 August 2024 |website=The European Southern Observatory (ESO)}}</ref>
|-
| [[Thirty Meter Telescope]] (TMT)
| $1400 million
| $3600 million<ref name=":0">{{Cite web |title=NSF board approves funding for just one of two proposed giant telescopes |url=https://www.science.org/content/article/nsf-board-approves-funding-just-one-two-proposed-giant-telescopes |access-date=29 August 2024 |website=Science (Science.org)}}</ref>
|-
| [[Giant Magellan Telescope]] (GMT)
| $1000 million 
|$2540 million<ref name=":0" />
|- style="background: #FFFFE0;"
| [[Large Binocular Telescope]] (LBT)
| $120 million 
|
|-
|}

==Projects==
{| class="infobox" style="width: 300px"
|-
|style="white-space: nowrap; background-color: #444;" |
{| cellpadding=0 cellspacing=0
|-
| colspan=2 |[[File:Latest Rendering of the E-ELT.jpg|x96px|European Extremely Large Telescope (39.2m), Chile|link=European Extremely Large Telescope]]&thinsp;[[File:Top view of tmt complex.jpg|x96px|Thirty Meter Telescope (30m), Hawaii|link=Thirty Meter Telescope]]
|-
| rowspan=2 |[[File:Giant Magellan Telescope - artist's concept.jpg|x106px|Giant Magellan Telescope (24.5m), Chile|link=Giant Magellan Telescope]]
| [[File:LargeBinoTelescope NASA.jpg|Large Binocular Telescope (2x 8.4m, 11.8m)|link=Large Binocular Telescope|x53px]][[File:KeckTelescopes-hi.png|x53px|Keck Observatory (2x10m), Hawaii|link=Keck Observatory]]
|-
|[[File:Paranal opendome.jpg|x52px|Very Large Telescope (4x 8.2m, VLTI)|link=Very Large Telescope]][[File:Grantelescopio.jpg|Gran Telescopio Canarias (10.4m)|link=Gran Telescopio Canarias|x52px]]
|}
|-
|Extremely large telescopes:
* [[Extremely Large Telescope]] (top left)
* [[Thirty Meter Telescope]] (top right)
* [[Giant Magellan Telescope]] (bottom left)
Compared to the [[Large Binocular Telescope|LBT]], [[Keck Observatory|Keck]], [[Very Large Telescope|VLT]], and [[Gran Telescopio Canarias|GTC]]
|}

There were several telescopes in various stages in the 1990s and early 2000s, and some developed into construction projects.

;Under construction
* ELT:<ref>{{Cite web|url=http://www.eso.org/public/teles-instr/elt/|title=Elt &#124; Eso}}</ref> [[European Extremely Large Telescope|Extremely Large Telescope]]
* GMT:<ref>{{Cite web|url=https://gmto.org/|title=Giant Magellan Telescope|website=Giant Magellan Telescope}}</ref> [[Giant Magellan Telescope]]

;Funded construction
* TMT:<ref>{{Cite web|url=https://www.tmt.org/|title=TMT International Observatory|website=TIO}}</ref> [[Thirty Meter Telescope]]

;Projects
Some of these projects have been cancelled, or merged into ongoing extremely large telescopes.
* GSMT:<ref>{{Cite web|url=http://www.aura-nio.noao.edu/|title=AURA NIO: Home|date=October 19, 2006|archive-url=https://web.archive.org/web/20061019105956/http://www.aura-nio.noao.edu/|archive-date=2006-10-19}}</ref> [[Giant Segmented Mirror Telescope]], merged into TMT
* OWL:<ref>{{Cite web|url=https://www.eso.org/sci/facilities/eelt/owl/|title=The ESO 100-m OWL optical telescope concept|website=www.eso.org}}</ref> [[Overwhelmingly Large Telescope]], passed over in favor of ELT
* VLOT:<ref>{{Cite web|url=http://www.hia-iha.nrc-cnrc.gc.ca/VLOT/|title=VLOT {{)}} The Very Large Optical Telescope for CI am Canada|date=May 2, 2009|archive-url=https://web.archive.org/web/20090502050342/http://www.hia-iha.nrc-cnrc.gc.ca/VLOT/|archive-date=2009-05-02}}</ref> [[Very Large Optical Telescope]], merged into TMT
* LAT:<ref>{{Cite web |url=http://astrosun.tn.cornell.edu/atacama |title=Atacama |access-date=2006-12-13 |archive-date=2003-12-02 |archive-url=https://web.archive.org/web/20031202212639/http://astrosun.tn.cornell.edu/atacama |url-status=dead }}</ref> [[Large Atacama Telescope]]
* EURO50:<ref>{{Cite web|url=http://www.astro.lu.se/~torben/euro50/|title=Euro50|date=December 14, 2006|archive-url=https://web.archive.org/web/20061214083436/http://www.astro.lu.se/~torben/euro50/|archive-date=2006-12-14}}</ref> [[European 50-metre Telescope]], merged into ELT
* LPT:<ref>{{Cite web|url=http://serweb.oamp.fr/denis/elt/ngcfht_may01.html|title=LPT {{!}} Large Petal Telescope|date=November 23, 2006|archive-url=https://web.archive.org/web/20061123185947/http://serweb.oamp.fr/denis/elt/ngcfht_may01.html|archive-date=2006-11-23}}</ref> [[Large Petal Telescope]]
* Magellan 20: merged into [[Giant Magellan Telescope|GMT]]
* HDRT:<ref>https://home.ifa.hawaii.edu/users/kuhn/tmp.html%25|title= 22 March 2002. </ref> High Dynamic Range Telescope
* JELT:<ref>http://optik2.mtk.nao.ac.jp/futureplan/eltproje.htm{{Dead link|date=December 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> [[Japanese ELT Project]]; Japan joined the TMT project in 2008
* CELT:<ref>{{Cite web|url=http://celt.ucolick.org/|title=index.html|date=June 5, 2002|archive-url=https://web.archive.org/web/20020605200532/http://celt.ucolick.org/|archive-date=2002-06-05}}</ref> [[California Extremely Large Telescope]], became/merged into TMT
* MAXAT:<ref name="MAXAT">{{cite web |title=MAXAT – the Maximum Aperture Telescope |url=http://www.gemini.edu/science/maxat/ |archive-url=https://web.archive.org/web/20070706082910/http://www.gemini.edu/science/maxat/ |archive-date=6 July 2007}}</ref> [[Maximum Aperture Telescope]]

==See also==
* [[List of largest optical reflecting telescopes]]
* [[List of telescope types]]

== References ==
{{Reflist}}

== External links ==
* {{commons category-inline}}
* [https://web.archive.org/web/20060825102344/http://www.aao.gov.au/instrum/ELT/Workshop/ Australian National Workshop on Extremely Large Telescopes (ELTs)]
* [http://www.astro-opticon.org/fp5/ELT.html The OPTICON ELT Working Group] a [[Europe]]-wide research project
* [https://web.archive.org/web/20120213001211/http://www.roe.ac.uk/ukatc/projects/elt/ The science case for Extremely Large Telescopes (ELTs)] from the [[Royal Observatory, Edinburgh]]
* [https://web.archive.org/web/20200602141340/http://the-colossus.com/ Colossus Telescope]

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[[Category:Reflecting telescopes]]