[cover story, Lowell Observer, Fall 1996]

The Navy Precision Optical Interferometer (NPOI) at Lowell Observatory has achieved a major milestone. At the June 1996 meeting of the American Astronomical Society in Madison, Wisconsin, astronomer David Mozurkewich of the Naval Research Lab presented results of NPOI observations of the bright binary star Zeta 1 Ursa Majoris.

This binary consists of two gravitationally linked stars so close together in the sky that neither the largest conventional groundbased telescopes nor the Hubble Space Telescope can resolve them as two separate stars. High-resolution imaging, however, is the forte of interferometry, and NPOI easily resolved the two components of Zeta 1 Ursa Majoris for a period of six weeks as they made nearly a complete orbit around their common center of mass. The photograph above shows three of the images of the system produced by NPOI.

The images were produced using only three of the siderostats near the very center of the Y-shaped NPOI array. Work continues towards extending each arm of the array out to the full 250-meter length planned. Under contract with the Naval Research Laboratory, Lowell Observatory has collaborated on the design of the interferometer and has been responsible for overseeing construction of the specialized infrastructure for what is expected to be the world's most advanced optical interferometer. This infrastructure includes six buildings, 1.5 miles of acess roads, power distribution, and over 420 concrete piers containing some 1,800 bolts. According to Lowell's project manager, Nat White, "the most challenging aspect of the construction project was that each bolt and pier had to be within one-quarter inch of its design position across the entire 15-acre interferometer site.

When the full span of the array is built and the entire interferometer completed, NPOI will be able to image stars much closer together than the components of Zeta 1 Ursa Majoris, and even resolve features on the disks of individual stars. Such measurements will permit astronomers to measure the masses and distances of stars and to understand dynamical properties of their surfaces. The interferometer will also be ideal for discovering solar systems in the process of formation around other stars. In the meantime, siderostats near the center of the array will continue to be used for observations of binary stars and precise measurements of stellar positions. The Navy Precision Optical Interferometer is a joint project of the Naval Research Laboratory, the U.S. Naval Observatory, and Lowell Observatory.