James Clerk Maxwell telescope. View of the open observatory housing the James Clerk Maxwell Telescope (JCMT). JCMT is a 15 metre telescope designed for observations at millimetre and sub- millimetre wavelengths (between radio & infrared). The telescope has a primary or main mirror, paraboloid in shape, composed of 276 adjustable panels. Radiation is focused from this mirror onto a small secondary hyperboloid mirror held above the main mirror by a metal frame. Infrared detectors then receive the signal from a cabin located below the main mirror. JCMT is located at 4200 metres altitude on the summit of the dormant volcano Mauna Kea in Hawaii.