Extremely large telescope

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

An extremely large telescope (ELT) is an astronomical observatory featuring a telescope with an aperture for its primary mirror from 20 metres up to 100 metres across,[1] when discussing reflecting telescopes of optical wavelengths including ultraviolet (UV), visible, and near infrared wavelengths. Among many planned capabilities, ELTs are planned to increase the chance of finding Earth-like planets around other stars.[2] 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. 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.[3][4]

Although ELT 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.


# Image Name Aperture (m) Area (m²) Primary mirror Altitude (m) First
Notes Refs
5 European Extremely Large Telescope
39.3 978 798 × 1.45 m
hexagonal (f/1)
3060 2024 Under construction: Cerro Armazones Obs., Chile [5][6][7]
4 Thirty Meter Telescope
30 655 492 × 1.45 m
hexagonal (f/1)
4050 TBD On hold:[8] Mauna Kea Obs., Hawaii [3][9]
3 Giant Magellan Telescope
24.5 368 7 × 8.4 m
circular (f/0.71)
2516 2021 Under construction: Las Campanas Obs., Chile;
4 mirrors cast (4/7 M1)
2 Large Binocular Telescope
11.8 equiv area
22.8 equiv detail limit
111 2 × 8.4 m
3221 2008 largest non-segmented mirrors;
Located on Mount Graham in Arizona
1 Gran Telescopio Canarias
10.4 74 36 × 1.9 m
2275 2008 Largest single mirror.
Located on Roque de los Muchachos Obs. in the Canary Islands
Note: Aperture of LBT: the baseline is obtained via aperture synthesis

The Keck Observatory (2 x 10 m) and the Very Large Telescope, of the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile, is also of note, with 4 × 8.2 m and 4 × 1.8 m, all on separate mounts but in one building for interferometry.


Possible budget figures, which are estimates and can vary over time.

Name Cost
(est USD)
European Extremely Large Telescope (E-ELT) $1192 million €1055 million (Euros)
Thirty Meter Telescope (TMT) $1200 million
Giant Magellan Telescope (GMT) $700 million
Large Binocular Telescope (LBT) $120 million
Gran Telescopio Canarias (GTC) $147 million €130 million (Euros)


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

Funded construction

Some of these projects have been cancelled, or merged into ongoing ELTs.

See also


  1. As A Skeleton Science Case For Extremely Large (20m–100m) Ground-based Telescopes (ELTs) and first section of ELT Roadmap, PDF
  2. Jha, Alok (5 August 2006). "Extremely Large Telescope could reveal secrets of life, the universe and everything". The Guardian.
  3. 1 2 "Thirty Meter Telescope Construction Proposal" (PDF). TMT Observatory Corporation. 2007-09-12: 29. Retrieved 2009-07-24.
  4. 1 2 "Chapter 6: Optics" (PDF). GMT Conceptual Design Report. GMT Consortium. pp. 6–3. Retrieved 2008-04-02.
  5. http://www.eso.org/ eso1419 — Organisation Release, Groundbreaking for the E-ELT, 19 June 2014
  6. Govert Schilling - Europe Downscales Monster Telescope to Save Money ( 14 June 2011) - Science Insider
  7. http://www.eso.org/public/astronomy/teles-instr/e-elt_num.html
  8. Stewart, Burnett, Colin M., John (October 14, 2016). "Hawaii Supreme Court voids Thirty Meter Telescope permit". Oahu Publications. West Hawaii Today. Retrieved 19 December 2015.
  9. Thirty Meter Telescope timeline page, TMT Observatory Project, retrieved 2010-10-12
  10. http://sen.com, Elizabeth Howell, Giant telescope gets $20m funding boost as design takes shape, 29 December 2014
  11. "Large Binocular Telescope Achieves First Binocular Light" (Press release). Large Binocular Telescope Corporation. 2008-02-28.
  12. "Giant Canary Islands telescope captures first light". CBCnews. CBC. 16 July 2007. Retrieved 24 July 2013.
  13. 1 2 ELT

External links

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