Stellar Interferometry Methods

Abstract

Stellar interferometry may be defined as the art of utilizing interference effects for improving the angular resolution of stellar observations. Resolution is improved in two ways: (a) interferometers attached to conventional telescopes can approach the diffraction limit of resolution, which is not accessible to ordinary observation with large telescopes owing to atmospheric turbulence, and (b) because interferometers can have base­ lines appreciably larger than the size of monolithic mirrors, their resolu­ tion is subject to improvement in proportion. Improving the angular resolution of optical observations has been a dream of astronomers from the time of William Herschel, when it became clear that image sharpness was limited by the atmosphere rather than by telescope optics. Following Fizeau's idea of observing through a pair of holes, ways in which the atmospheric limitation could be overcome were brilliantly demonstrated by Stephan (1873), Michelson ( 1920), and more recently Hanbury Brown (1974). Not only were these pioneers able to approach the diffraction limit in large telescopes, but they also succeeded in reducing this limit further by increasing the baseline spans beyond the size of telescope apertures. Pease's observation of the brightest red stars provided the first measure­ ments of stellar angular diameters. With still higher resolution, R. Hanbury Brown and 1. Davis were able to resolve and measure the angular diameters of the 32 brightest blue stars in the southern hemisphere. For both red and blue stars, angular diameters provide direct informa­ tion on how much the object deviates from a black-body source. Indeed, black bodies of given temperature, or color, and apparent magnitude have a predictable apparent size that can be checked against observation. Diameter excess may indicate an absorbing envelope, or temperature