Session 16 - Astrometry.
Display session, Wednesday, January 07
Exhibit Hall,

## [16.03] The Navy Prototype Optical Interferometer (NPOI) is Operational

J. T. Armstrong, D. Mozurkewich, T. A. Pauls, L. J. Rickard (NRL), J. A. Benson, H. M. Dyck, N. M. Elias II, A. R. Hajian, C. A. Hummel, D. J. Hutter, K. J. Johnston (USNO), N. M. White (Lowell Obs.)

The Navy Prototype Optical Interferometer (NPOI) has been operating since May 1996 with three simultaneous baselines (18.9, 22.2, and 37.5 m) and 32 simultaneous wavelength channels covering \lambda\lambda 450-850 nm. One scan yields three squared visibility amplitudes (V^2) and one closure phase for each of the 32 channels. With these wavelengths and the current baselines, the NPOI can currently resolve binaries with separations >2 mas, and can observe limb darkening on stellar disks with diameters \sim 5 mas by measuring V^2 beyond the first null of the visibility function. Current observational programs include: \beginitemize \item Binary star observations (Hummel et al., this meeting; see also Benson et al.\ 1997, AJ, 114,1221) \item Limb-darkened angular diameter measurements (Pauls et al., this meeting; see also Hajian et al.\ 1997, ApJ, accepted) \item Astrometric measurements (Hutter et al., this meeting) \enditemize

The NPOI when complete will comprise two subarrays. The astrometric subarray will have four elements, baselines of 19 to 38 m, and an array metrology system for high-precision astrometry. The imaging subarray can include any combination of fixed astrometric elements and moveable imaging elements, and will have baseline lengths of 2 to 437 m. The current configuration consists of three of the four astrometric elements, and includes most of the array metrology. The fourth astrometric element and the first of the imaging elements will be commissioned during the winter of 1997/98. Extending the imaging subarray to its full size and the development of extended delay lines are planned for 1998 and 1999. A more complete description of the NPOI is given by Armstrong et al.\ (1997, ApJ, accepted).