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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 (see poster by Armstrong et al.). The AI will be in operation on Anderson Mesa, near Flagstaff, AZ, in early 1994.
The AI is being built using the experience gained from the Mark III Interferometer on Mt. Wilson, CA. Measurements of stellar positions by the Mark III, made over a five-year period, have uncertainties of 10 to 20 milliarcseconds (mas).
The AI incorporates several significant design improvements which will greatly increase its astrometric accuracy over that of the Mark III. These improvements include multi-way beam combination and dispered fringe detection techniques, an increase in the number of siderostats from two to four, an extensive laser metrology system to fully monitor the motions of the baselines in three dimensions, increased siderostat aperture, state-of-the-art delay lines, and greater detector sensitivity.
The combination of these improvements will permit the AI to produce highly accurate catalogs of star positions on a nearly inertial frame. The AI will establish, by mid 1996, a catalog of the order of one thousand stars with statistical and systematic errors of 1 to 3 mas. The AI, with a planned operational lifetime of several decades, will maintain the HIPPARCOS reference frame by improving the measured proper motions of thousands of the brighter HIPPARCOS stars.
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