Extremely Large Telescopes
An extremely large telescope (ELT) is an astronomical observatory featuring a telescope with an aperture of more than 20 m diameter, 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. 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, 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. 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 achieve much more light collecting ability, 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 and TMT.
|Name||Image||Aperture (m)||Area (m²)||Primary mirror||Altitude (m)|| Cost
|European Extremely Large Telescope (E-ELT)
||39.3||978||798 × 1.45 m hexagonal (f/1)||3060||€1055 million||2022||Site chosen: Cerro Armazones, Chile|
|Thirty Meter Telescope (TMT)||30||655||492 × 1.45 hexagonal (f/1)||4050||$1200 million||2022||Site chosen: Mauna Kea, Hawaii|
|Giant Magellan Telescope (GMT)||
|24.5||368||7 × 8.4 m circular (f/0.71)||2516||$700 million||2020||Site chosen: Las Campanas, Chile; 3 mirrors cast (3/7 M1), 1 polished|
|Large Binocular Telescope (LBT)||22.8*||111||2 × 8.4 m circular||3221||$120 million||2008|| Largest Binocular; largest non-segmented mirrors; |
Located on Mount Graham in Arizona
|Gran Telescopio Canarias (GTC)||10.4||74||36 × 1.9 m hexagonal||2275||€130 million||2008|| Largest single mirror.|
Located on Roque de los Muchachos Observatory 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.
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
- Giant Magellan Telescope
- European Extremely Large Telescope
- Thirty Meter Telescope
Some of these projects are completed, or merged into ongoing ELTs.
- GSMT: Giant Segmented Mirror Telescope, merged into TMT
- Overwhelmingly Large Telescope;passed over in favor of E-ELT
- VLOT: Very Large Optical Telescope, merged into TMT
- LAT: Large Atacama Telescope
- EURO50: European 50-metre Telescope;merged into the E-ELT.
- LPT: Large Petal Telescope
- JELT: Japanese ELT Project. Japan joined the TMT project in 2008.
- California Extremely Large Telescope; CELT became/merged into TMT project.
- List of largest optical reflecting telescopes
- Overwhelmingly Large Telescope
- Gran Telescopio Canarias
- Australian National Workshop on Extremely Large Telescopes (ELTs)
- Europe-wide research project
- Royal Observatory, Edinburgh