Working with a space-based optical interferometer - HST Fine Guidance Sensor 3 small field astrometry

Space-based interferometry already exists. We describe our experiences with on-orbit calibration and scientific observations with Fine Guidance Sensor 3 (FGS3), a white-light interferometer aboard Hubble Space Telescope. Our goal, 1 millisecond of are precision small-field astrometry, has been achieved, but not without significant challenges. These included a mechanically noisy on-orbit environment, the self-calibration of FGS 3, and significant temporal changes in our instrument. Solutions included a denser set of drift check stars for each science observation, fine-tuning exposure times, overlapping field observations and analyses for calibration, and a continuing series of trend-monitoring observations. HST FGS 3 will remain a competitive astrometric tool for faint targets in crowded fields and for faint small separation binaries until the advent of large-aperture, ground-based and longer-baseline space-based interferometers.