SIM PlanetQuest will measure star positions to an accuracy of a few microarcseconds using precise white light
fringe measurements. One challenge for the SIM observation scenario is "star confusion," where multiple stars
are present in the instrument field of view. This is especially relevant for observing dim science targets because
the density of number of stars increases rapidly with star magnitude. We study the effect of star confusion on
the SIM astrometric performance due to systematic fringe errors caused by the extra photons from the confusion
star(s). Since star confusion from multiple stars may be analyzed as a linear superposition of the effect from single
star confusion, we quantify the astrometric errors due to single star confusion surveying over many spectral types,
including A0V, F0V, K5III, and M0V, and for various visual magnitude differences. To the leading order, the
star confusion effect is characterized by the magnitude difference, spectral difference, and the angular separation
between the target and confusion stars.
Strategies for dealing with star confusion are presented. For example, since the presence of additional sources
in the field of view leads to inconsistent delay estimates from different channels, with sufficient signal to noise
ratio, the star confusion can be detected using chi-square statistics of fringe measurements from multiple spectral
channels. An interesting result is that the star confusion can be detected even though the interferometer cannot
resolve the separation between the target and confusion stars when their spectra are sufficiently different. Other
strategies for mitigating the star confusion effect are also discussed.