Astrometric “Imaging” of Crowded Stellar Fields with Only TwoSIMPointings

The Space Interferometry Mission (SIM) will observe sources in crowdedfields. Recent work has shown that source crowding can induce significantpositional errors in SIM's astrometric measurements, even for targets manymagnitudes brighter than all other crowding sources. Here we investigatewhether the spectral decomposition of the fringe pattern may be used todisentangle the overlapping fringes from multiple blended sources, effectivelyby performing synthesis imaging with two baselines. We find that spectrallydispersed fringes enable SIM to identify and localize a limited number of fieldsources quite robustly, thereby removing their effect from SIM astrometry andreducing astrometry errors to near photon noise levels. We simulate SIMmeasurements of the LMC, and show that (a) SIM astrometry will not be corruptedby blending and (b) extremely precise imaging of mildly crowded fields may beperformed using only two orthogonal baseline orientations, allowingmicroarcsecond positional measurements. We lastly illustrate the method'spotential with the example of astrometric microlensing, showing that SIM's massand distance measurements of lenses will be untainted by crowding.Comment: 9 pages, 9 figures, uses emulateapj5 (included), submitted to Ap