Cambridge Optical Aperture Synthesis Telescope

COAST stands in the flat fenland near Cambridge, a cluster of small telescopes mounted on steel beams that stretch up to 100 metres apart. The array forms an optical interferometer, combining starlight from separate mirrors to mimic one giant telescope. Its resolving power reaches one thousandth of an arcsecond, far sharper than images from the Hubble Space Telescope. The instrument was designed by John E. Baldwin and first proved it could photograph the surfaces of stars beyond the Sun. Earlier work on the William Herschel Telescope had shown only lower-resolution views, so COAST’s clear pictures marked a step forward. Today it is run by the Cavendish Astrophysics Group from the Mullard Radio Astronomy Observatory. Visitors walk along gravel paths between the telescope units, each on its own concrete pier. The steel frames and mirror supports stand against the wide Cambridgeshire sky, silent except for the occasional hum of electronics. In the control shed nearby, astronomers watch computer screens that display the interference patterns of starlight. The scene is practical and unadorned, a working observatory rather than a showpiece.