extremely large telescope

The term extremely large telescope (ELT) has commonly been used to mean a telescope considerably larger than the 10-meter-class telescopes built in the 1990s and 2000s, using 20 meters (or something similar) as a defining minimum aperture. As of 2019, 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 is to use 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, planned, for Mauna Kea, Hawaii.

Some past and speculative concepts:

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

The E-ELT has taken the name the Extremely Large Telescope aka the ELT.