extremely large telescope

The term extremely large telescope (ELT) is commonly used to mean a telescope considerably larger than the 10-meter-class telescopes that were built during the 1990s and 2000s. Defining minimum apertures used are on the order of 20 meters. As of 2024, some are funded and under construction but none are complete and the largest operating telescopes still have apertures of on the order of 10 meters (though some can use multiple such units in concert). A typical ELT design uses a segmented mirror with adaptive optics. Current plans and construction:

• Giant Magellan Telescope (GMT) - 24.5 m, under construction, at Las Campanas, Chile.
• European Extremely Large Telescope (E-ELT) - 39.3 m, under construction, at Cerro Armazones, Chile.
• Thirty Meter Telescope (TMT) - 30 m, under construction, for Mauna Kea, Hawaii.

Some past and speculative concepts:

• California Extremely Large Telescope (CELT).
• Giant Segmented Mirror Telescope (GSMT).
• Overwhelmingly Large Telescope (OWL).
• Colossus Telescope.

The name Extremely Large Telescope (ELT) was adopted for the E-ELT in 2017.