James Clerk Maxwell telescope. Side view of the primary mirror and framework of the James Clerk Maxwell Telescope (JCMT). JCMT is a 15 metre telescope designed for observations at millimetre and sub-millimetre wavelengths (between radio and infrared). The primary or main mirror (at top) is composed of 276 adjustable panels. Radiation is focused from this mirror onto a small secondary mirror which reflects the signal to infrared detectors in a cabin located below the main mirror (at bottom). The telescope is seen housed in its observatory. JCMT is located at 4200 metres altitude on the summit of the dormant volcano Mauna Kea in Hawaii.