extremely large telescope

Freely steerable radio telescopes with diameters up to 100 m have been in operation since the 1970s. 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. Despite the size of extremely large telescope designs, 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.

• # — Image — Name — data-sort-type="number" | Aperture (m) — Area (m 2) — Primary mirror — Altitude (m) — First light — Notes — Refs • 1 — — Extremely Large Telescope (ELT) — 39.5 — 978 — 798 × 1.45 m hexagonal 1 — 3060 — 2029 — Under construction at Cerro Armazones Obs., Chile • 2 — — Thirty Meter Telescope (TMT) — 30.0 — 655 — 492 × 1.45 m hexagonal 1 — 4050 — Construction approved at Mauna Kea Obs. in Hawaii, USA, halted as of September 2019 due to protests • 3 — — Giant Magellan Telescope (GMT) — 24.5 — 368 — 7 × 8.4 m circular 0.71 — 2516 — 2030s — Under construction at Las Campanas Obs., Chile • 4 — — Large Binocular Telescope (LBT) — 11.8 (equiv. area) 22.8 (equiv. detail limit) — 111 — 2 × 8.4 m circular — 3221 — 2008 — Largest non-segmented mirrors. Located on Mount Graham, Arizona, USA • Note: Aperture of LBT: the baseline is obtained via aperture synthesis. • Illustration — The domes of the ELT, the TMT and the GMT compared to other well-known telescopes

Possible budget figures, which are estimates and can vary over time. For construction costs, it is recommended to estimate the cost of a giant telescope with the relation cost ∝ D 2.7. • Name — Cost (est. USD) — Alternate • Extremely Large Telescope (ELT) — $1590 million — €1300 million • Thirty Meter Telescope (TMT) — $1400 million — $3600 million • Giant Magellan Telescope (GMT) — $1000 million — $2540 million • Large Binocular Telescope (LBT) — $120 million

There were several telescopes in various stages in the 1990s and early 2000s, and some developed into construction projects. • Under construction • ELT: Extremely Large Telescope • GMT: Giant Magellan Telescope • Funded construction • TMT: Thirty Meter Telescope • Projects Some of these projects have been cancelled, or merged into ongoing extremely large telescopes. • GSMT: Giant Segmented Mirror Telescope, merged into TMT • OWL: Overwhelmingly Large Telescope, passed over in favor of ELT • VLOT: Very Large Optical Telescope, merged into TMT • LAT: Large Atacama Telescope • EURO50: European 50-metre Telescope, merged into ELT • LPT: Large Petal Telescope • Magellan 20: merged into GMT • HDRT: High Dynamic Range Telescope • JELT: Japanese ELT Project; Japan joined the TMT project in 2008 • CELT: California Extremely Large Telescope, became/merged into TMT • MAXAT: Maximum Aperture Telescope