OBSS is a instrument to determine the positions, proper motions, parallaxes, and photometry of 1,000,000,000 stars from 5th to 20th visual magnitude with 50 microarcseconds accuracy at 9th visual magnitude. It will greatly expand upon the observations of the successful Hipparcos satellite. It will help define positions for SIM grid stars, identify candidate stars for SIM and TPF, and complement the observational program of the pointed SIM instrument, which can observe about 10,000 stars at 4 microarcseconds. SIM science center and OBSS personnel will be involved in data reduction procedures that have some commonality. There will be early identification of relevant techniques and reusable software. OBSS will give an additional temporal baseline for SIM observations.

OBSS vastly surpasses ground-based astrometric programs. The best wide-field accuracies from the ground are achieved by optical interferometers, which can reach 1 milliarcsecond (mas) accuracy for about 50 stars per night. There are no astrometric optical interferometers in the southern hemisphere. Narrow field accuracies of hundreds of microarcseconds can be achieved by interferometers and special instruments on very large telescopes for only a small number of stars. Ground-based survey instruments using charge coupled devices (CCDs) can at best achieve 25 mas relative accuracies. Thus, there is no current means of achieving a full-sky survey of millions of stars at microarcseconds accuracies other than by a space instrument.

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