The Thirty Meter Telescope (TMT) is a proposed extremely large telescope (ELT) that has become controversial due to its planned location on Mauna Kea, on the island of Hawaii, land which is sacred to Native Hawaiian culture and religion. The TMT would become the largest visible-light telescope on Mauna Kea.

Scientists have been considering ELTs since the mid-1980s. In 2000, astronomers considered the possibility of a telescope with a light-gathering mirror larger than 20 meters in diameter.

The US National Academy of Sciences recommended a 30-meter telescope be the focus of U.S. interests, seeking to see it built within the decade. Scientists at the University of California and Caltech began development of a design that would eventually become the TMT, consisting of 492 segmented mirrors with nine times the power of the Keck Observatory.

Due to its immense light-gathering power and the optimal observing conditions which exist atop Mauna Kea, the TMT would enable astronomers to conduct research which is infeasible with current instruments. The TMT is designed for near-ultraviolet to mid-infrared (0.31 to 28 μm wavelengths) observations, featuring adaptive optics to assist in correcting image blur. The TMT will be at the highest altitude of all the proposed ELTs. The telescope has government-level support from several nations.

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