Extremely large telescope
An extremely large telescope (ELT) is an astronomical observatory featuring an optical telescope with an aperture for its primary mirror from 20 metres up to 100 metres across, when discussing reflecting telescopes of optical wavelengths including ultraviolet (UV), visible, and near infrared wavelengths. Among many planned capabilities, extremely large telescopes are planned to increase the chance of finding Earth-like planets around other stars. Telescopes for radio wavelengths can be much bigger physically, such as the 300 metres aperture fixed focus radio telescope of the Arecibo Observatory. Freely steerable radio telescopes with diameters up to 100 metres have been in operation since the 1970s.
Image: Latest Rendering of the E ELT
Image: Top view of tmt complex
Image: Giant Magellan Telescope artist's concept
The domes of the ELT, the TMT and the GMT compared to other well-known telescopes
A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric. They are used as objectives for large reflecting telescopes. To function, all the mirror segments have to be polished to a precise shape and actively aligned by a computer-controlled active optics system using actuators built into the mirror support cell.
Early segmented mirror, built out of 61 hexagonal segments in 1952.
SALT's segmented mirror
The Keck II telescope showing the segmented primary mirror
Mirror segments made of beryllium