The James Clerk Maxwell Telescope (JCMT) is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. The telescope is near the summit of Mauna Kea at 13,425 feet (4,092 m). Its primary mirror is 15 metres (16.4 yards) across: it is the largest single-dish telescope that operates in submillimetre wavelengths of the electromagnetic spectrum (far-infrared to microwave). Scientists use it to study the Solar System, interstellar dust and gas, and distant galaxies.
The JCMT started operations in 1987, and was funded until February 2015 by a partnership between the United Kingdom and Canada, and the Netherlands. It was operated by the Joint Astronomy Centre and was named in honour of mathematical physicist James Clerk Maxwell. In March 2015 the operation of the JCMT was taken over by the East Asian Observatory.
The JCMT has the second-largest telescope mirror on Mauna Kea (the largest is the VLBA antenna).
The telescope was combined with the Caltech Submillimeter Observatory next to it, to form the first submillimetre interferometer. This success was important in pushing ahead the construction of the later Submillimeter Array and the Atacama Large Millimeter Array interferometers.