Extremely large telescope

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European Extremely Large Telescope (39.2m), Chile Thirty 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 some existing telescopes (hover with mouse over image):

 · European Extremely Large Telescope (top left)
 · Thirty Meter Telescope (top right)
 · Giant Magellan Telescope (bottom left)
 · Comparison: Large Binocular Telescope, Keck Observatory,    Very Large Telescope, Gran Telescopio Canarias

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 the TMT[5] and 2024 for the E-ELT.[6]

Image Name Aperture (m) Area (m²) Primary mirror Altitude (m) First
Notes Refs
Latest Rendering of the E-ELT.jpg European Extremely Large Telescope
39.3 978 798 × 1.45 m
hexagonal (f/1)
3060 2024 Under construction: Cerro Armazones Obs., Chile [7][8][6]
Top view of tmt complex.jpg Thirty Meter Telescope
30 655 492 × 1.45 m
hexagonal (f/1)
4050 2022 Under construction: Mauna Kea Obs., Hawaii [3][5]
Giant Magellan Telescope - artist's concept.jpg Giant Magellan Telescope
24.5 368 7 × 8.4 m
circular (f/0.71)
2516 2021 Site chosen: Las Campanas Obs., Chile;
3 mirrors cast (3/7 M1), 1 polished
LBT 2.png Large Binocular Telescope
12.8 111 2 × 8.4 m
3221 2008 largest non-segmented mirrors;
Located on Mount Graham in Arizona
Grantelescopio.jpg 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.

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) $1407 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) $173 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]


External links[edit]