Extremely Large Telescopes

Extremely Large Telescopes

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, 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 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[5] and TMT.[6]

Name Image Aperture (m) Area (m²) Primary mirror Altitude (m) Cost
First light Notes
European Extremely Large Telescope (E-ELT)
39.3 978[7] 798 × 1.45 m hexagonal (f/1) 3060[5] €1055 million 2022[5] Site chosen: Cerro Armazones, Chile
Thirty Meter Telescope (TMT) 30 655[3] 492 × 1.45 hexagonal (f/1) 4050 $1200 million 2022[6] Site chosen: Mauna Kea, Hawaii
Giant Magellan Telescope (GMT)

24.5 368[4] 7 × 8.4 m circular (f/0.71) 2516 $700 million 2020[8] 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[9] 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[10] 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.

See also


External links

  • Australian National Workshop on Extremely Large Telescopes (ELTs)
  • Europe-wide research project
  • Royal Observatory, Edinburgh