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Extremely large telescope

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Comparison of nominal sizes of apertures of the above extremely large telescopes and some notable optical telescopes

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 300 metres (330 yards) aperture fixed focus radio telescope of the Arecibo Observatory (now defunct). Freely steerable radio telescopes with diameters up to 100 metres (110 yards) 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[edit]

# Image Name Aperture (m) Area (m2) Primary mirror Altitude (m) First
Notes Refs
1 Extremely Large Telescope
39.3 978 798 × 1.45 m
hexagonal f/1
3060 2028 Under construction at Cerro Armazones Obs., Chile [6][7][8]
2 Thirty Meter Telescope
30.0 655 492 × 1.45 m
hexagonal f/1
4050 2030s Construction approved at Mauna Kea Obs. in Hawaii, USA, halted as of September 2019 due to protests [4][9][10][11]
3 Giant Magellan Telescope
24.5 368 7 × 8.4 m
circular f/0.71
2516 late 2020s Under construction at Las Campanas Obs., Chile;
6/7 mirrors cast
4 Large Binocular Telescope
(equiv. area)
(equiv. detail limit)
111 2 × 8.4 m
3221 2008 Largest non-segmented mirrors.
Located on Mount Graham, Arizona, USA
Note: Aperture of LBT: the baseline is obtained via aperture synthesis.


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:[14]

Name Cost
(est. USD)
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


European Extremely Large Telescope (39.2m), ChileThirty Meter Telescope (30m), Hawaii
Giant Magellan Telescope (24.5m), Chile Large Binocular Telescope (2x 8.4m, 11.8m)Keck Observatory (2x10m), Hawaii
Very Large Telescope (4x 8.2m, VLTI)Gran Telescopio Canarias (10.4m)
Extremely large telescopes:

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
Funded construction

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

See also[edit]


  1. ^ As A Skeleton Science Case For Extremely Large (20m–100m) Ground-based Telescopes (ELTs) and first section of ELT Roadmap Archived 2015-05-18 at the Wayback Machine, PDF
  2. ^ Overbye, Dennis (8 March 2024). "Good News and Bad News for Astronomers' Biggest Dream - The National Science Foundation takes a step (just one) toward an "extremely large telescope."". The New York Times. Archived from the original on 8 March 2024. Retrieved 8 March 2024.
  3. ^ Jha, Alok (5 August 2006). "Extremely Large Telescope could reveal secrets of life, the universe and everything". The Guardian.
  4. ^ a b "Thirty Meter Telescope Construction Proposal" (PDF). TMT Observatory Corporation. 2007-09-12: 29. Archived from the original (PDF) on 2016-03-24. Retrieved 2009-07-24. {{cite journal}}: Cite journal requires |journal= (help)
  5. ^ a b "Chapter 6: Optics" (PDF). GMT Conceptual Design Report. GMT Consortium. pp. 6–3. Retrieved 2008-04-02.
  6. ^ "Groundbreaking for the E-ELT". 19 June 2014.
  7. ^ Schilling, Govert (14 June 2011). "Europe Downscales Monster Telescope to Save Money". Science Insider.
  8. ^ "The E-ELT in numbers". Archived from the original on 2009-08-21. Retrieved 2009-08-11.
  9. ^ Thirty Meter Telescope timeline page, TMT Observatory Project, archived from the original on 2010-09-25, retrieved 2010-10-12
  10. ^ TMT Timeline, accessed February 11, 2018
  11. ^ "Hawaii top court approves controversial Thirty Meter Telescope". BBC News. 31 October 2018.
  12. ^ Howell, Elizabeth (29 December 2014). "Giant telescope gets $20m funding boost as design takes shape". Sen. Archived from the original on 17 July 2021. Retrieved 8 January 2015.
  13. ^ "Large Binocular Telescope Achieves First Binocular Light" (Press release). Large Binocular Telescope Corporation. 2008-02-28. Archived from the original on 2008-03-10.
  14. ^ Stepp, Larry; Daggert, Larry; Gillett, Paul. "Estimating the costs of extremely large telescopes" (PDF). National Optical Astronomy Observatory.
  15. ^ "Elt | Eso".
  16. ^ "Giant Magellan Telescope". Giant Magellan Telescope.
  17. ^ "TMT International Observatory". TIO.
  18. ^ "AURA NIO: Home". October 19, 2006. Archived from the original on 2006-10-19.
  19. ^ "The ESO 100-m OWL optical telescope concept". www.eso.org.
  20. ^ "VLOT } The Very Large Optical Telescope for CI am Canada". May 2, 2009. Archived from the original on 2009-05-02.
  21. ^ "Atacama". Archived from the original on 2003-12-02. Retrieved 2006-12-13.
  22. ^ "Euro50". December 14, 2006. Archived from the original on 2006-12-14.
  23. ^ "LPT | Large Petal Telescope". November 23, 2006. Archived from the original on 2006-11-23.
  24. ^ https://home.ifa.hawaii.edu/users/kuhn/tmp.html%25%7Ctitle= 22 March 2002.
  25. ^ http://optik2.mtk.nao.ac.jp/futureplan/eltproje.htm[permanent dead link]
  26. ^ "index.html". June 5, 2002. Archived from the original on 2002-06-05.
  27. ^ "MAXAT – the Maximum Aperture Telescope". Archived from the original on 6 July 2007.

External links[edit]