Current Status of the Navy Prototype Optical Interferometer

The Navy Prototype Optical Interferometer (NPOI), located at Anderson Mesa, Arizona, is a joint project of the U.S. Naval Observatory and the Naval Research Laboratory, and is operated in cooperation with Lowell Observatory. The NPOI consists of two nearly coincident arrays of siderostats. One array consists of four stationary astrometric siderostats with baselines between 18 m and 35 m, while the other contains six movable imaging siderostats with baselines up to 435 m. The arrays share six fast vacuum delay lines, each supplying up to 35 m of optical delay, as well as much of the feed system and a novel multi-beam combiner and fringe-detection system operating over the range of 450-900 nm. The astrometric array, equipped with 50-cm siderostats and an extensive laser metrology system to monitor baseline motion, will have a limiting visual magnitude of ~ 10 and will produce catalogs of star positions accurate to a few mas. The imaging array, in its final configuration, will have 30 stations and six long delay lines (in series with the fast delay lines to provide additional delay). It will be capable of imaging stellar surfaces and circumstellar material, and measuring stellar diameters and binary separations to tens of mu as. The NPOI achieved ``first light'' (first fringe detection on stars) on 28 October 1994 and reliable fringe tracking on a 35 m east-west baseline in September 1995. Progress towards achieving multi-baseline operations and closure-phase measurements will be reviewed.