Corner cube interferometer using extended sources

The corner cube interferometer employing extended sources requires very good transverse alignment of the retroreflector if the optical path difference is not to vary across the field of view. It is shown in this paper that this problem can be eliminated by the addition of a plane mirror to the interferometer forming a double pass system. For a single pass instrument, it is shown that the sensitivity to alignment errors may be exploited to obtain a powerful method of monitoring in three dimension the motion of a point otherwise inaccessible. Details of the expected performance of this method are derived.