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 larger physically, such as the 100 meter (109 yards) aperture on the Green Bank Telescope for radio wavelengths, but different wavelengths like radio have different capabilities.
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.
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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 one.[5]
| Name | Image | aperture (m) | Equiv. ap. area | Area (m2) | M1 Mirror | Note | Altitude |
|---|---|---|---|---|---|---|---|
| European Extremely Large Telescope (E-ELT) Image credit ESO |
39.3 | 39.3 | 978 m2[6] | 798 × 1.45 m hexagonal segments (f/1) | Site chosen: Cerro Armazones, Chile | 3060 m[5] | |
| Thirty Meter Telescope (TMT) | 30 | 30 | 655 m2[3] | 492 × 1.45 hexagonal segments (f/1) | Site chosen: Mauna Kea, Hawaii | 4050 m | |
| Giant Magellan Telescope (GMT) | 24.5 | 21.4 | 368 m2[4] | 7 × 8.4 m mirrors | Site chosen: Las Campanas, Chile; 1 mirror cast (1/7 M1) | 2516 m | |
| Large Binocular Telescope (LBT) | 22.8* | 11.7 | 111 m2 | 2 × 8.4 m M1 mirrors; 1 mount (2 × M1) | Largest Binocular; largest non-segmented mirrors; First bino-light 2008 | 3221 m | |
| Gran Telescopio Canarias (GTC) | 10.4 | 10.4 | 74 m2 | 36 × 1.9 m hexagonal segments for M1 | Largest single mirror; Fully Operational 2009 | 2275 m |
*The LBT telescope baseline is via aperture synthesis.
The Very Large Telescope 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.
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.
Some of these projects are completed, or merged into ongoing ELTs.