Crowded-Field Astrometry with the Space Interferometry Mission - I. Estimating the Single-Measurement Astrometric Bias Arising from Confusion

摘要

The accuracy of position measurements on stellar targets with the futureSpace Interferometry Mission (SIM) will be limited not only by photon noise andby the properties of the instrument (design, stability, etc.) and the overallmeasurement program (observing strategy, reduction methods, etc.), but also bythe presence of other "confusing" stars in the field of view (FOV). We use asimple "phasor" model as an aid to understanding the main effects of this"confusion bias" in single observations with SIM. This analytic model has beenimplemented numerically in a computer code and applied to a selection oftypical SIM target fields drawn from some of the Key Projects already acceptedfor the Mission. We expect that less than 1% of all SIM targets will bevulnerable to confusion bias; we show that for the present SIM design,confusion may be a concern if the surface density of field stars exceeds 0.4star/arcsec^2. We have developed a software tool as an aid to ascertaining thepossible presence of confusion bias in single observations of any arbitraryfield. Some a priori knowledge of the locations and spectral energydistributions of the few brightest stars in the FOV is helpful in establishingthe possible presence of confusion bias, but the information is in general notlikely to be available with sufficient accuracy to permit its removal. Wediscuss several ways of reducing the likelihood of confusion bias in crowdedfields. Finally, several limitations of the present semi-analytic approach arereviewed, and their effects on the present results are estimated. The simplemodel presented here provides a good physical understanding of how confusionarises in a single SIM observation, and has sufficient precision to establishthe likelihood of a bias in most cases.