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The instrument described here is fundamentally a stellar interferometer employing fiber optic links to combine very high resolution with high sensitivity. In a typical (Michelson's) stellar interferometer, outriggers and mirrors are attached to an astronomical telescope so that objects with small angular subtense (binary stars, star diameters) can he observed by each of two mirrors separated by an adjustable distance. The spacing of the resulting interference pattern depends on the angle between the two incident beams, and the visibility or contrast of the frinse pattern depends on the separation of the mirrors and the angular subtense of the astronomical object.(1) Binary stars with a subtense a observed at a mean wavelength A show a steadily decreasing visibility with increasing mirror separation until the separation s is s = λ/2a (1) where the visibility is zero. These conventional stellar instruments have a number of shortcomings. (a) The mechanical structure required to support the outriggers, etc. is interferometrically unstable resulting in random changes in the fringe pattern which increase the difficulty of quantitatively assessing fringe visibility. This instability limits the practical size of the interferometer to less than 10 meters. (h) A large telescope is required, even though its full aperture is not utilized. (c) The relatively small amount of light collected by the outrigger mirrors is spread over a relatively large number of fringes, and is thus wasteful of light. (d) Turbulence with frequency components higher than the eye can temporally resolve causes rapid fringe motions which impair the observer's ability to estimate fringe contrast. Other stellar instruments or measurement techniques, such as intensity interferometry(2) and speckle interferometry(3), avoid some of these problems but require bright sources. The instrument described here avoids these problems and collects substantially larger quantities of light, permitting much fainter objects to he measured. The experimental measurements described here demonstrate the feasibility of the instrument.
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