Alignment of optical delay lines for long-baseline stellar interferometry

The problem of aligning an optical delay line for use in long-baseline stellar interferometry is discussed. In particular, the development of the Navy Prototype Optical Interferometer alignment system is described. Rapid mirror alignment must be performed with sufficient precision that beam shear is limited to a few millimeters over an optical path length that may exceed 800 m. Two possible alignment algorithms are presented. The first is a null-seeking servo where the mirrors are adjusted to minimize the sum of their angular alignment errors. The second method utilizes a priori knowledge of the mirror separations to minimize the total shear. A number of time-dependent and time-independent errors that affect alignment and alignment stability are also discussed.