MULTICHANNEL OPTICAL APERTURE SYNTHESIS IMAGING OF ZETA(1)-URSAE-MAJORIS WITH THE NAVY PROTOTYPE OPTICAL INTERFEROMETER

We have used the Navy Prototype Optical Interferometer (NPOI) to obtai n the first multichannel optical aperture synthesis images of a star. We observed the spectroscopic binary zeta(1) Ursae Majoris at 6 to 10 milliarcseconds separation during seven nights, using three interferom etric baselines and 19 spectral channels (lambda lambda 520-850 nm) of the NPOI. After editing, a typical 90 sec scan yielded fringe visibil ities at 50 spatial frequencies and closure phases at 15 wavelengths. Three to five scans were obtained each night. The separations and posi tion angles are in good agreement with the visual orbit obtained with the Mark III interferometer (Hummel et al. 1995, AJ, 110, 376) but sho w small systematic difference that can be used to improve the orbit. T he closure phase data provide a sensitive measure of the magnitude dif ference between the components. These results demonstrate the power of broad-band interferometric observations for fast imaging and the util ity of vacuum delay lines for simultaneous observations over a wide ba nd. These observations are the first to produce simultaneous visibilit ies and closure phases with a separate-aperture optical interferometer , and the second to produce closure phase images, following the result s from COAST reported by Baldwin et al. (1996, A&A, 306, L13). The ang ular resolution here is the highest ever achieved at visual wavelength s, exceeding by an order of magnitude the best thus far achieved by an y single-aperture optical telescope. We generated complex visibilities and closure phases (the data types commonly used in radio interferome try) from the optical data and used standard radio interferometry tech niques to produce these images. However, the fundamental observables o f optical interferometry, the squared visibility amplitude and the clo sure phase, require the development of new analysis techniques. (C) 19 97 American Astronomical Society.