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COAST, the Cambridge Optical Aperture Synthesis Telescope, is a multi-element optical astronomical interferometer with baselines of up to 100 metres, which uses aperture synthesis to observe stars with angular resolution as high as one thousandth of one arcsecond (producing images with much higher resolution than can be obtained using individual telescopes such as the Hubble Space Telescope). The principal limitation is that COAST can only image bright stars. COAST was the first long-baseline interferometer to obtain high-resolution images of the surfaces of stars other than our sun (although the surfaces of other stars had previously been imaged at lower resolution using Aperture Masking Interferometry on the William Herschel Telescope).
The COAST array was conceived by John E. Baldwin, and is operated by the Cavendish Astrophysics Group. It is situated at the Mullard Radio Astronomy Observatory in Cambridgeshire, England.