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 meter (109 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 ELTs are large, they have smaller apertures than the aperture synthesis on many large optical interferometers. However, they have much more light collecting ability, along with other advantages.
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