COAST, for Cambridge Optical Aperture Synthesis Telescope, is an optical interferometer located at Mullard Radio Astronomy Observatory, near Cambridge, England. The telescope is not currently used, at least not for the imaging that was intended. It uses aperture synthesis, with a baseline of up to 67 meters. Light is collected in 40 cm diameter reflector telescopes, and routed to a building containing the optical instruments to manipulate the beams and route them through Michelson interferometers. It first imaged in 1995. It had four telescopes, later expanded to five, with plans to expand with an additional telescope, extending longest baseline to 100 meters.