USNO Astrometric Interferometer

The U.S. Naval Observatory Astrometric Interferometer (AI) is the dedicated astrometric subarray of the new Navy Prototype Optical Interferometer (NPOI) at Lowell Observatory, which is being built in collaboration with the Naval Research Laboratory. The Naval Research Laboratory is constructing the imaging subarray of the NPOI, the `Big Optical Array' (BOA). The AI will be in operation on Anderson Mesa, near Flagstaff, Arizona, in the spring of 1994. The AI was built using the experience gained from the Mark III Interferometer on Mt. Wilson, CA, which demonstrated `proof of concept' of wide-angle astrometry by a long baseline optical interferometer. The AI incorporates four siderostats that are located in a Y-shaped configuration, and features a full-array laser metrology system to monitor baseline motion. The AI shares with the BOA state-of-the-art delay lines and a real-time zero- order fringe tracking system. The AI will have a limiting magnitude of 10 and will produce star positions accurate to a few milliarcseconds (mas). With a planned operational lifetime of several decades, this instrument will be capable of maintaining the HIPPARCOS reference frame through repeated observations, yielding improved proper motions of thousands of the brighter HIPPARCOS stars.