OSI: Orbiting Stellar Interferometer for Astrometry and Imaging

OSI, Orbiting Stellar Interferometer, is a concept for a space interferometer mission with both astrometric and imaging goals: 10 μarc sec astrometric accuracy for 16th mag objects in 100 sec of integration time and for 20th mag objects in 1 h, and 5 mas resolution aperture synthesis imaging. The instrument consists of 6 siderostats arranged as 3 co-linear interferometers on a single structure; two interferometers are used for attitude control, and the third for science observations. Delay lines are used for path- length equalization, allowing a large field of view. Available baselines range from 1 to 20 m, with ~50 cm clear apertures. High astrometric precision is obtained with a laser metrology system which monitors the instrument baselines, i.e., the siderostat positions, as well as the delay-line positions. Mission and flight system studies resulted in an implementation which would use an Atlas HAS for insertion into a 900 km sun- synchronous orbit. The total spacecraft mass was ~3600 kg (including contingencies), and the structure would fold in order to fit within a standard launch shroud. Some of the science possible with OSI includes the astrometric detection of planets and the high resolution imaging of Seyfert II narrow-line regions.