Introduction to COAST
The Instrument
The quest for high angular resolution at optical wavelengths has prompted the development of a variety of techniques to overcome atmospheric seeing at astronomical sites. These include speckle interferometry, triple correlation, aperture masking, adaptive optics, and, with the advent of the Hubble Space Telescope, satellite observations.
Unfortunately these methods, when used on existing single telescopes, are in principle limited to resolutions no better than about 10 milliarcseconds. While this represents an improvement in resolution by a factor of around 40 over that achieved under typical observing conditions, it is still inadequate for many astronomical problems.
It is clear that long-baseline interferometry is the only realistic method for achieving higher resolutions.
The Cambridge Optical Aperture Synthesis Telescope (COAST) was planned as a coherent array of four telescopes operating in the red and near infra-red, using Michelson interferometry on baselines of up to 100m to give images with a resolution down to 1 milliarcsecond. It was the first instrument of its kind to exploit the techniques of aperture synthesis and closure phase at optical or infra-red wavelengths, producing the first images from an optical aperture synthesis telescope on 95/09/13 and 95/09/28.
The total cost of the telescope was around бё850,000.
The Objectives
The astronomical objectives of COAST are to provide very high resolution images of a wide variety of stellar systems down to a red magnitude of 10, including: stellar surfaces, the envelopes of pre-main sequence stars, pulsating variables, circumstellar shells, compact planetary nebulae and close binary and multiple systems.