Confusion field effects on angle tracking camera centroiding estimates for SIM PlanetQuest

SIM PlanetQuest will be the first space-baaed interferometer and will allow astrometric measurements that are several hundred times more accurate than the previous missions operating at optical wavelengths. SIM promises to achieve microarcsecond accuracy for astrometry on objects as faint as 20th visual magnitude. One of the challenges is to achieve this accuracy at these dim levels, in the presence of even dimmer stars inside the SIM's FOV. Therefore it is important to investigate the effects of "confusion" fields on astrometrie performance for SIM. This study will look at effects of Angle Tracking Camera performance for SIM in presence of a crowded or confusion field near a target star. This will lead to a study that predicts SIM astrometric performance in a crowded field. Centroid displacements due to any perturbation including a crowded field cause a shift in the fringe and reduce visibility and performance. In this work we will devise an estimator to estimate the bias in the centroid of SIM Angle Tracking camera in presence of a crowded field. This analysis will examine pointing accuracy performance by estimating Angle Tracking centroid under different parameter variations that affect performance. These parameters are number of neighboring stars, stellar types, and angular separation as well as their relative brightness.