Extremely large telescope

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] 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.[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.

For the specific observatory by this name, see Extremely Large Telescope.
Comparison of nominal sizes of apertures of the above extremely large telescopes and some notable optical telescopes

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 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 Aperture (m) Area (m²) Primary mirror Altitude (m) First
light		 Notes Refs
1 Extremely Large Telescope
(ELT)		 39.3 978 798 × 1.45 m
hexagonal f/1		 3060 2025 Under construction at Cerro Armazones Obs. in Chile [5][6][7]
2 Thirty Meter Telescope
(TMT)		 30.0 655 492 × 1.45 m
hexagonal f/1		 4050 2027 Construction approved at Mauna Kea Obs. in Hawaii, halted as of September 2019 due to protests [3][8][9][10]
3 Giant Magellan Telescope
(GMT)		 24.5 368 7 × 8.4 m
circular f/0.71		 2516 2029 Under construction at Las Campanas Obs. in Chile;
4/7 mirrors cast		 [4][11]
4 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 in Arizona		 [12]
5 Gran Telescopio Canarias
(GTC)		 10.4 74 36 × 1.9 m
hexagonal		 2275 2008 Largest single mirror.
Located at Roque de los Muchachos Obs. in the Canary Islands		 [13]
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, measure 4 × 8.2 m and 4 × 1.8 m, all on separate mounts but in one building for interferometry.

  • The domes of the ELT, the TMT and the GMT compared to other, well known telescopes

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[14]:

Name Cost
(est. USD)		 Alternate
Extremely Large Telescope (ELT) $1245 million €1055 million
Thirty Meter Telescope (TMT) $1400 million
Giant Magellan Telescope (GMT) $1000 million
Large Binocular Telescope (LBT) $120 million
Gran Telescopio Canarias (GTC) $153 million €130 million

Projects
 
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
Projects

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

See also

References
  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. Jha, Alok (5 August 2006). "Extremely Large Telescope could reveal secrets of life, the universe and everything". The Guardian.
  3. "Thirty Meter Telescope Construction Proposal" (PDF). TMT Observatory Corporation. 2007-09-12: 29. Retrieved 2009-07-24. Cite journal requires |journal= (help)
  4. "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. "Archived copy". Archived from the original on 2009-08-21. Retrieved 2009-08-11.CS1 maint: archived copy as title (link)
  8. Thirty Meter Telescope timeline page, TMT Observatory Project, retrieved 2010-10-12
  9. TMT Timeline, accessed February 11, 2018
  10. https://www.bbc.com/news/world-us-canada-46046864
  11. http://sen.com, Elizabeth Howell, Giant telescope gets $20m funding boost as design takes shape, 29 December 2014
  12. "Large Binocular Telescope Achieves First Binocular Light" (Press release). Large Binocular Telescope Corporation. 2008-02-28. Archived from the original on 2008-03-10.
  13. "Giant Canary Islands telescope captures first light". CBCnews. CBC. 16 July 2007. Retrieved 24 July 2013.
  14. Stepp, Daggert, Gillett, Larry, Larry, Paul. "Estimating the costs of extremely large telescopes" (PDF). National Optical Astronomy Observatory.CS1 maint: multiple names: authors list (link)
  15. ELT
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