Michelson stellar interferometer

Any one of a series of instruments constructed by A. A. Michelson (1852-1931) in an attempt to measure, by means of interferometry, the diameters of stars, none of which can be resolved directly by ground-based telescopes. In the simplest form, the telescope objective is covered by a screen containing two holes. If the star were a point source of light, the image would appear as a circular ring pattern crossed by dark fringes. For a source of finite angular diameter a and light of wavelength l, the fringes disappear when the separation of the holes is 1.22 l/a, a result that can be used to determine a. Though the theory is simple, the practical difficulties are very great. In the most successful experiment, Michelson used the 2.54-metre (100-inch) Hooker Telescope at Mount Wilson and mounted a pair of mirrors on racks to achieve a large, variable separation. (In fact, with this design, the size of the telescope objective is irrelevant. Michelson used the Hooker Telescope because it was rigid enough to bear the weight of the extra structures.) The use of the instrument was limited by the fact that no more than a handful of stars were bright enough. The angular diameters of six stars were measured, notably that of Betelgeuse.