Abstract

The effects of non-normal-incidence optical phase shifts at the surfaces of mirrors in the arms of stellar interferometers are considered. An explicit expression is given for the wavelength-dependant loss in fringe visibility which results from measureable phase shifts between the perpendicular and parallel reflected components of unpolarized radiation. This loss occurs as a result of the generally different reflection to-pologies in the various arms of an interferometer. An optical design method is presented which eliminates these potentially degrading effects. This method essentially says that the radiation in each arm should experience reflections having the same sequence of direction cosines between the point where the beam is divided and the point where it is combined. We also show that other useful properties of interferometers are obtained by following this rule. Wavelength-dependent phase-shift measurements on real mirrors are presented, allowing numerical evaluation of the degradation factor for any optical configuration. Various optical configurations are shown to illustrate the effect, including new COSMIC (Coherent Optical System of Modular Interferometric Collectors) designs.

© 1988 Optical Society of America