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An extremely large telescope (ELT) is a class of telescope with an aperture of more than 20 m diameter^[1] when discussing reflecting telescopes of optical wavelengths including UV, Visible, and Near infrared wavelengths. Telescopes for other wavelengths can be much larger such as the 100 meter (109 yards) aperture on the Green Bank Telescope for radio wavelengths. ELTs are planned to increase the chance of finding earth-like planets around other stars.^[2]

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 ELTs are large, they have smaller apertures than the aperture synthesis on many large optical interferometers. However, they have much more light collecting ability, along with other advantages.

ELTs

Top three ELTs that are funded with two smaller but completed large telescopes for comparison (background yellow).

Name	aperture (m)	Equiv. ap. area	Area (m²)	M1 Mirror	Note
European Extremely Large Telescope (E-ELT) Image credit ESO	42	42	1300 m² ^[5]	984 x 1.45 m hexagonal segments (f/1)	Largest with funding
Thirty Meter Telescope (TMT)	30	30	655 m²^[3]	492 x 1.45 hexagonal segments (f/1)	Site Chosen
Giant Magellan Telescope (GMT)	24.5	21.4	368 m²^[4]	7 x 8.4 m mirrors	Site chosen, 1 mirror cast
Large Binocular Telescope (LBT)	22.8*	11.7	111 m²	2 x 8.4 m M1 mirrors; 1 mount (2 x M1)	Largest Binocular; largest non-segmented mirrors; Fully operational 2008
Gran Telescopio Canarias (GTC)	10.4	10.4	74 m²	36 x 1.9 m hexagonal segments for M1	Largest single mirror; Fully Operational 2009

*The LBT telescope baseline is via aperture synthesis.

The Very Large Telescope is also of note, with 4 x 8.2 m, 4 x 1.8, and 1 x 2.61, all on separate mounts but in one building for interferometry.

Projects

There were several telescopes in various stages of design or construction by the late 1990s and early 2000s, but only some developed into construction projects.

Funded construction

Projects

Some of these projects are completed, or merged into ongoing ELTs.

References

^ See title of http://www.astro-opticon.org/fp5/skelcase.html and section 1 of http://www.aao.gov.au/instrum/ELT/ELTroadmap040917.pdf
^ http://www.guardian.co.uk/science/2006/aug/05/spaceexploration.universe Guardian August 2005
^ ^a ^b Thirty Meter Telescope Construction Proposal (PDF), TMT Observatory Corporation, 2007-09-12, p. 29, retrieved 2009-07-24
^ ^a ^b "Chapter 6: Optics". GMT Conceptual Design Report. GMT Consortium. p. 6-3. Retrieved 2008-04-02. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
^ http://www.eso.org/public/astronomy/teles-instr/e-elt_num.html

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

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	An '''extremely large telescope''' (ELT) is a class of [[telescope]] with a ~~aperature~~ of more than 20 m diameter<ref>See title of http://www.astro-opticon.org/fp5/skelcase.html and section 1 of http://www.aao.gov.au/instrum/ELT/ELTroadmap040917.pdf</ref> when discussing reflecting telescopes of optical wavelengths including UV, Visible, and Near infrared wavelengths. Telescopes for other wavelengths can be much larger such as the 100 meter (109 yards) aperture on the [[Green Bank Telescope]] for radio wavelengths. ELTs are planned to increase the chance of finding earth-like planets around other stars.<ref> http://www.guardian.co.uk/science/2006/aug/05/spaceexploration.universe Guardian August 2005 </ref>		An '''extremely large telescope''' (ELT) is a class of [[telescope]] with an aperture of more than 20 m diameter<ref>See title of http://www.astro-opticon.org/fp5/skelcase.html and section 1 of http://www.aao.gov.au/instrum/ELT/ELTroadmap040917.pdf</ref> when discussing reflecting telescopes of optical wavelengths including UV, Visible, and Near infrared wavelengths. Telescopes for other wavelengths can be much larger such as the 100 meter (109 yards) aperture on the [[Green Bank Telescope]] for radio wavelengths. ELTs are planned to increase the chance of finding earth-like planets around other stars.<ref> http://www.guardian.co.uk/science/2006/aug/05/spaceexploration.universe Guardian August 2005 </ref>

	These telescopes have a number of features in common, in particular the use of a [[segmented mirror\|segmented primary mirror]] (similar to the existing [[Keck telescopes]]), and the use of high-order [[adaptive optics]] systems.<ref name=constr>{{Citation \|url=http://www.tmt.org/news/TMT-Construction%20Proposal-Public.pdf \|publisher=TMT Observatory Corporation \|page=29 \|title=Thirty Meter Telescope Construction Proposal \|date=2007-09-12 \|accessdate=2009-07-24}}</ref><ref name=design_optics>{{Cite book \|title=GMT Conceptual Design Report \|chapter=Chapter 6: Optics \|url=http://www.gmto.org/CoDRpublic \|chapterurl=http://www.gmto.org/codrfolder/GMT-ID-01467-Chapter_6_Optics.pdf \|page=6-3 \|accessdate=2008-04-02 \|publisher=GMT Consortium}}</ref> See also the [[List of largest optical reflecting telescopes]] for other large finished telescopes.		These telescopes have a number of features in common, in particular the use of a [[segmented mirror\|segmented primary mirror]] (similar to the existing [[Keck telescopes]]), and the use of high-order [[adaptive optics]] systems.<ref name=constr>{{Citation \|url=http://www.tmt.org/news/TMT-Construction%20Proposal-Public.pdf \|publisher=TMT Observatory Corporation \|page=29 \|title=Thirty Meter Telescope Construction Proposal \|date=2007-09-12 \|accessdate=2009-07-24}}</ref><ref name=design_optics>{{Cite book \|title=GMT Conceptual Design Report \|chapter=Chapter 6: Optics \|url=http://www.gmto.org/CoDRpublic \|chapterurl=http://www.gmto.org/codrfolder/GMT-ID-01467-Chapter_6_Optics.pdf \|page=6-3 \|accessdate=2008-04-02 \|publisher=GMT Consortium}}</ref> See also the [[List of largest optical reflecting telescopes]] for other large finished telescopes.