THE NAVY PROTOTYPE OPTICAL INTERFEROMETER

We describe the Navy Prototype Optical Interferometer (NPOI), a joint project of the Naval Research Laboratory (NRL) and the US Naval Observ atory (USNO) in cooperation with Lowell Observatory. The NPOI has rece ntly begun operations at the Lowell Observatory site near Flagstaff, A rizona, obtaining its first images, of a binary star, in 1996 May and June and its first limb-darkening observations during 1996 November to 1997 February. This paper gives an overview of the NPOI, including th e characteristics of optical interferometry that affect its design. Th e NPOI includes subarrays for imaging and for astrometry. The imaging subarray consists of six moveable 50 cm siderostats feeding 12 cm aper tures, with baseline lengths from 2.0 to 437 m. The astrometric subarr ay consists of four fixed 50 cm siderostats feeding 12 cm apertures (3 5 cm apertures to be installed in 1998), with baseline lengths from 19 m to 38 m. The shared back end covers 450-850 nm in 32 channels. The NPOI features vacuum feed and delay systems, active group-delay fringe tracking, and a high degree of automation. The astrometric subarray a lso includes an extensive site laser metrology system to measure the m otions of the siderostats with respect to one another and to the bedro ck. For imaging stellar surfaces, arrays with equal spacing between el ements are superior to arrays that have been laid out to optimize (u, v) coverage and that therefore have unequal spacing. The imaging subar ray of the NPOI provides a number of equally spaced configurations wit h linear scales at ratios of approximate to 1.64. Unequally spaced con figurations are available for a variety of other imaging programs. Coh erence across either type of imaging configuration is maintained by '' phase bootstrapping'': the phases on the longest baselines, on which f ringes may be too weak to track, are stabilized by tracking fringes on the shortest baselines. In principle, the four elements of the astrom etric subarray provide enough independent baselines to solve for stell ar positions and the array geometry simultaneously while observing eac h of 11 stars only once. The anticipated magnitude limit is 7 mag or b etter with 12 cm apertures and average seeing; with 35 cm apertures, w e expect the limit to be one or more magnitudes fainter. The anticipat ed wide-angle astrometric precision of the NPOI is approximate to 2 ma s. The best angular resolution of the imaging subarray will be approxi mate to 200 mu as. Our experience with the Mark III interferometer sug gests that we will be able to measure stellar diameters as small as 20 0 mu as with 1% precision and binary star separations as small as rho approximate to 65 mu as (for Delta m approximate to 0 mag) or rho appr oximate to 200 mu as (for Delta m approximate to 3-4 mag). With its la rge bandwidth and phase bootstrapping, the imaging subarray should be able to make images greater than or similar to 10 resolution elements across the disks of nearby late-type stars.