Extremely large telescope

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OWL concept art

An extremely large telescope (ELT) is an astronomical observatory featuring a telescope with an aperture of more than 20 m diameter,[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 other wavelengths can be much bigger physically, such as the 100 metres (110 yards) aperture on the Green Bank Telescope for radio wavelengths.

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] See also the List of largest optical reflecting telescopes for other large finished telescopes.

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.


The top three ELTs that are funded with two smaller but completed large telescopes for comparison (background yellow). In the early 2000s, all three targeted completion in 2018, although this slipped to 2022 for E-ELT[5] and TMT.[6]

Image Name Aperture (m) Area (m²) Primary mirror Altitude (m) First
Latest Rendering of the E-ELT.jpg European Extremely Large Telescope (E-ELT) 39.3 978[7] 798 × 1.45 m
hexagonal (f/1)
3060[5] 2022[5] Under construction: Cerro Armazones Obs., Chile
Top view of tmt complex.jpg Thirty Meter Telescope (TMT) 30 655[3] 492 × 1.45 m
hexagonal (f/1)
4050 2022[6] Under construction: Mauna Kea Obs., Hawaii
Giant Magellan Telescope - artist's concept.jpg Giant Magellan Telescope (GMT) 24.5 368[4] 7 × 8.4 m
circular (f/0.71)
2516 2020[8] Site chosen: Las Campanas Obs., Chile;
3 mirrors cast (3/7 M1), 1 polished
LBT 2.png Large Binocular Telescope (LBT) 22.8* 111 2 × 8.4 m
3221 2008[9] largest non-segmented mirrors;
Located on Mount Graham in Arizona
Grantelescopio.jpg Gran Telescopio Canarias (GTC) 10.4 74 36 × 1.9 m
2275 2008[10] Largest single mirror.
Located on Roque de los Muchachos Obs. in the Canary Islands

*The LBT baseline is obtained via aperture synthesis.
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, 4 × 1.8, and 1 × 2.61, all on separate mounts but in one building for interferometry. See also Keck Observatory

Comparison of nominal sizes of primary mirrors of the above extremely large telescopes and some notable optical telescopes (click for detail)


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

Name Cost
(est USD)
European Extremely Large Telescope (E-ELT) $1353 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) $167 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 are completed, or merged into ongoing ELTs.

See also[edit]


  1. ^ See title of http://www.astro-opticon.org/fp5/skelcase.html and section 1 of http://www.aao.gov.au/instrum/ELT/ELTroadmap040917.pdf
  2. ^ Jha, Alok (5 August 2006). "Extremely Large Telescope could reveal secrets of life, the universe and everything". The Guardian. 
  3. ^ a b "Thirty Meter Telescope Construction Proposal". TMT Observatory Corporation. 2007-09-12. p. 29. Retrieved 2009-07-24. 
  4. ^ a b "Chapter 6: Optics". GMT Conceptual Design Report. GMT Consortium. pp. 6–3. Retrieved 2008-04-02. 
  5. ^ a b c Govert Schilling - Europe Downscales Monster Telescope to Save Money ( 14 June 2011) - Science Insider
  6. ^ a b Thirty Meter Telescope timeline page, TMT Observatory Project, retrieved 2010-10-12 
  7. ^ http://www.eso.org/public/astronomy/teles-instr/e-elt_num.html
  8. ^ Joe Palca (2012-01-14), "Mega Mirror To Power Massive New Telescope", All Things Considered, NPR, retrieved 2012-03-23 
  9. ^ "Large Binocular Telescope Achieves First Binocular Light" (Press release). Large Binocular Telescope Corporation. 2008-02-28. 
  10. ^ "Giant Canary Islands telescope captures first light". CBCnews. CBC. 16 July 2007. Retrieved 24 July 2013. 
  11. ^ a b ELT

External links[edit]