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Space Telescope Science Institute
Points Mission Studies: Lessons for SIM

Robert D. Reasenberg, Robert W. Babcock, John F. Chandler, and James D. Phillips
Harvard-Smithsonian Center for Astrophysics
Cambridge MA

POINTS (Precision Optical INTerferometer in Space) measures the angle between two widely separated stars. The nominal bright-star measurement accuracy of 2 microarcsec is achieved in two minutes of observing two magnitude 8 stars. POINTS comprises a metrology system and a pair of independent Michelson stellar interferometers, each with a pair of 35 cm subapertures and a 2 m baseline. The angle between the baselines is adjustable over the range of 87 to 93º. A series of mission simulations over the past 15 years have elucidated the consequences of the single measurement accuracy and instrument architecture. For the grid-lockup and planet-detection simulations, we describe the work and discuss application to SIM.

The POINTS scientific mission is enhanced by a solar shield, which allows observation of stars as close as 10º from the Sun. The wide separation between simultaneously observed stars makes POINTS a global astrometric instrument and provides three major advantages: (1) reference stars can be anywhere in a 6º by 360º band (5% of the sky). Such a band includes about 80 stars as bright as visual mag 5. Integration time is not extended by the need to use the faint stars that would be expected in a small field. (2) Parallax determinations are absolute. There is no need to use "zero parallax objects" which, given the measurement accuracy, would be extragalactic and thus faint. (3) Measurements over the entire sky allow recalibration and bias estimation through 360º closure on a time scale of hours for an agile spacecraft free of severe pointing restrictions.

For simplicity and efficiency, we divide the target stars into two classes, "reference grid stars" and all others. Grid stars provide reference points for other targets, and are also science targets. In the nominal mission, redundant grid-star observations are performed quarterly to determine the stars' positions, proper motions, and parallaxes. We showed more than a decade ago that, if the grid stars are observed with sufficient redundancy, the grid "locks up;" after the observations are combined in a weighted least squares estimate of star positions, proper motions, and parallaxes, the uncertainty in the angle between any grid pair, whether directly observable or not, becomes of the order of the measurement uncertainty. A mission might use 300 stars for redundancy since some stars may eventually be shown to have properties that make them unsuitable for precision astrometry. For the grid stars, POINTS would make about 360 measurements per day and complete a set of grid measurements in 4.2 days.

We have used double blind Monte-Carlo mission simulations to study the planet-finding capabilities of POINTS and to determine the reliable detection threshold with a nominal observing program. If we demand a negligible probability of false alarms, then with our standard observing schedule, the detection threshold is a planetary signature with amplitude equal to the single-measurement observing precision; orbital elements can usually be determined.