FGS Astrometry with the Hubble Space Telescope
  This article describes astrometric science that can be done with the Hubble Space Telescope Fine Guidance Sensors and presents some recent science results. We outline how these data are modeled, acquired, and calibrated. We next show how the astrometer, FGS 3, works as an interferometer. Finally, we present a guide to the literature that provides additional detail for each item discussed in the article. 
G. Fritz Benedict