Array metrology system for an optical long-baseline interferometer

We describe the array metrology system of the Navy Prototype Optical Interferometer, a long-baseline interferometer whose purpose is to determine precise positions of bright stars and image the surfaces and circumstellar environments of stars and stellar systems. For the astrometric array of the NPOI to achieve its design goal of wide-angle astrometric precision of 1-3 milliarcseconds on 20 m baselines, an extensive laser metrology system is employed to measure the three-dimensional motions of those baselines with respect to an Earth-fixed reference system to an accuracy of approximately 100 nm. The array metrology system is described, along with its associated data analysis software, test results, and the application of those results to the analysis of astrometric data. It is shown that at the current stage of its development, the array metrology system is capable of monitoring the variations in the geometry of the astrometric array with micron-level precision.