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An extremely large telescope (ELT) is an astronomical observatory featuring a telescope with an aperture for its primary mirror from 20 metres up to 100 metres across,〔As (A Skeleton Science Case For Extremely Large (20m–100m) Ground-based Telescopes (ELTs) ) and first section of (ELT Roadmap ), PDF〕 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 radio wavelengths can be much bigger physically, such as the aperture fixed focus radio telescope of the Arecibo Observatory. Freely steerable radio telescopes with diameters up to 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. See also the List of largest optical reflecting telescopes for other large finished telescopes. Although ELT designs are large, 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. ==ELTs== The top three ELTs that are being funded, with two smaller but completed large telescopes for comparison (background yellow). In the early 2000s, all three targeted completion in 2018, although this slipped to 2022 for the TMT〔 and 2024 for the E-ELT.〔 The Keck Observatory (2 x 10 m) and the Very Large Telescope, of the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile, is also of note, with 4 × 8.2 m and 4 × 1.8 m, all on separate mounts but in one building for interferometry. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「extremely large telescope」の詳細全文を読む スポンサード リンク
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