Extremely large telescope explained

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,^{[1] [2]} when discussing reflecting telescopes of optical wavelengths including ultraviolet (UV), visible, and near infrared wavelengths. Among many planned capabilities, extremely large telescopes are planned to increase the chance of finding Earth-like planets around other stars.^[3] Telescopes for radio wavelengths can be much bigger physically, such as the 300m (1,000feet) aperture fixed focus radio telescope of the Arecibo Observatory (now defunct). Freely steerable radio telescopes with diameters up to 100m (300feet) have been in operation since the 1970s.

These telescopes have a number of features in common, in particular the use of a segmented primary mirror (similar to the existing Keck telescopes), and the use of high-order adaptive optics systems.^{[4] [5]}

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

List of telescopes

	Image	Name	data-sort-type="number"	Aperture (m)	Area (m^2)	Primary mirror	Altitude (m)	First light	Notes	Refs
1	align=left nowrap		Extremely Large Telescope (ELT)	39.3	978	798 × 1.45 m hexagonal 1	3060	2028	Under construction at Cerro Armazones Obs., Chile	^[6] ^[7] ^[8]
2	align=left nowrap		Thirty Meter Telescope (TMT)	30.0	655	492 × 1.45 m hexagonal 1	4050	2030s	Construction approved at Mauna Kea Obs. in Hawaii, USA, halted as of September 2019 due to protests	^[9] ^[10]
3	align=left nowrap		Giant Magellan Telescope (GMT)	24.5	368	7 × 8.4 m circular 0.71	2516	late 2020s	Under construction at Las Campanas Obs., Chile; 6/7 mirrors cast	^[11]
4	align=left nowrap		Large Binocular Telescope (LBT)	11.8 (equiv. area) 22.8 (equiv. detail limit)	111	2 × 8.4 m circular	3221	2008	Largest non-segmented mirrors. Located on Mount Graham, Arizona, USA	^[12]
Note: Aperture of LBT: the baseline is obtained via aperture synthesis.

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:^[13]

cost \varpropto D ^{2.7}

Name	Cost (est. USD)	Alternate
Extremely Large Telescope (ELT)	$1590 million	€1300 million
Thirty Meter Telescope (TMT)	$1400 million
Giant Magellan Telescope (GMT)	$1000 million
Large Binocular Telescope (LBT)	$120 million

Projects

Extremely large telescopes: Extremely Large Telescope (top left) Thirty Meter Telescope (top right) Giant Magellan Telescope (bottom left) Compared to the LBT, Keck, VLT, and GTC

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

Under construction

• ELT:^[14] Extremely Large Telescope
• GMT:^[15] Giant Magellan Telescope

Funded construction

• TMT:^[16] Thirty Meter Telescope

ProjectsSome of these projects have been cancelled, or merged into ongoing extremely large telescopes.

• GSMT:^[17] Giant Segmented Mirror Telescope, merged into TMT
• OWL:^[18] Overwhelmingly Large Telescope, passed over in favor of ELT
• VLOT:^[19] Very Large Optical Telescope, merged into TMT
• LAT:^[20] Large Atacama Telescope
• EURO50:^[21] European 50-metre Telescope, merged into ELT
• LPT:^[22] Large Petal Telescope
• Magellan 20: merged into GMT
• HDRT:^[23] High Dynamic Range Telescope
• JELT:^[24] Japanese ELT Project; Japan joined the TMT project in 2008
• CELT:^[25] California Extremely Large Telescope, became/merged into TMT
• MAXAT:^[26] Maximum Aperture Telescope

See also

• List of largest optical reflecting telescopes
• List of telescope types

External links

• • Australian National Workshop on Extremely Large Telescopes (ELTs)
• The OPTICON ELT Working Group a Europe-wide research project
• The science case for Extremely Large Telescopes (ELTs) from the Royal Observatory, Edinburgh
• Colossus Telescope

Notes and References

1. As A Skeleton Science Case For Extremely Large (20m–100m) Ground-based Telescopes (ELTs) and first section of ELT Roadmap, PDF
2. News: Overbye . Dennis . Dennis Overbye . Good News and Bad News for Astronomers’ Biggest Dream - The National Science Foundation takes a step (just one) toward an “extremely large telescope.” . 8 March 2024 . . live . https://archive.today/20240308112703/https://www.nytimes.com/2024/03/08/science/astronomy-extremely-large-telescope.html . 8 March 2024 . 8 March 2024 .
3. News: Extremely Large Telescope could reveal secrets of life, the universe and everything . . Alok . Jha . 5 August 2006.
4. Thirty Meter Telescope Construction Proposal . TMT Observatory Corporation . 29 . 2007-09-12 . 2009-07-24 . 2016-03-24 . https://web.archive.org/web/20160324230243/http://tmt.org/news/TMT-Construction%20Proposal-Public.pdf . dead .
5. Book: GMT Conceptual Design Report . Chapter 6: Optics . http://www.gmto.org/codrfolder/GMT-ID-01467-Chapter_6_Optics.pdf . 6–3 . 2008-04-02 . GMT Consortium.
6. Web site: Groundbreaking for the E-ELT . 19 June 2014.
7. News: Europe Downscales Monster Telescope to Save Money . Govert . Schilling . 14 June 2011 . Science Insider.
8. Web site: The E-ELT in numbers . 2009-08-11 . https://web.archive.org/web/20090821225427/http://www.eso.org/public/astronomy/teles-instr/e-elt_num.html . 2009-08-21 . dead .
9. https://www.tmt.org/page/timeline?category=Observatory+Construction TMT Timeline
10. News: Hawaii top court approves controversial Thirty Meter Telescope . 31 October 2018 . BBC News.
11. News: Elizabeth . Howell . Giant telescope gets $20m funding boost as design takes shape . 29 December 2014 . Sen . 8 January 2015 . 17 July 2021 . https://web.archive.org/web/20210717164947/https://sen.com/news/giant-telescope-gets-20m-funding-boost-as-design-takes-shape . dead .
12. Large Binocular Telescope Achieves First Binocular Light . 2008-02-28 . Large Binocular Telescope Corporation . dead . https://web.archive.org/web/20080310010642/http://medusa.as.arizona.edu/lbto/firstbinocularlight_press_release.htm . 2008-03-10.
13. Web site: Estimating the costs of extremely large telescopes . Larry . Stepp . Larry . Daggert . Paul . Gillett . National Optical Astronomy Observatory.
14. Web site: Elt | Eso.
15. Web site: Giant Magellan Telescope. Giant Magellan Telescope.
16. Web site: TMT International Observatory. TIO.
17. Web site: AURA NIO: Home. October 19, 2006. https://web.archive.org/web/20061019105956/http://www.aura-nio.noao.edu/. 2006-10-19.
18. Web site: The ESO 100-m OWL optical telescope concept. www.eso.org.
19. Web site: VLOT The Very Large Optical Telescope for CI am Canada. May 2, 2009. https://web.archive.org/web/20090502050342/http://www.hia-iha.nrc-cnrc.gc.ca/VLOT/. 2009-05-02.
20. Web site: Atacama . 2006-12-13 . 2003-12-02 . https://web.archive.org/web/20031202212639/http://astrosun.tn.cornell.edu/atacama . dead .
21. Web site: Euro50. December 14, 2006. https://web.archive.org/web/20061214083436/http://www.astro.lu.se/~torben/euro50/. 2006-12-14.
22. Web site: LPT Large Petal Telescope. November 23, 2006. https://web.archive.org/web/20061123185947/http://serweb.oamp.fr/denis/elt/ngcfht_may01.html. 2006-11-23.
23. https://home.ifa.hawaii.edu/users/kuhn/tmp.html%25|title= 22 March 2002.
24. http://optik2.mtk.nao.ac.jp/futureplan/eltproje.htm{{Dead link|date=December 2022 |bot=InternetArchiveBot |fix-attempted=yes }}
25. Web site: index.html. June 5, 2002. https://web.archive.org/web/20020605200532/http://celt.ucolick.org/. 2002-06-05.
26. Web site: MAXAT – the Maximum Aperture Telescope . https://web.archive.org/web/20070706082910/http://www.gemini.edu/science/maxat/ . 6 July 2007.