  The optical structure of several AGN has been studied using the Fine Guidance
Sensors (FGS) on the Hubble Space Telescope (HST). The FGSs are interferometric
devices which can resolve structure on scales of 20 milliarcsecs or less and
hence have the potential to improve on the resolution attainable by HST's
cameras. The FGSs produce interferometric fringes known as S-curves which are
related to the intensity profile of the object on the sky. These have been
analyzed using a simple model for the radial intensity distribution and
strength of the underlying background illumination of the observed objects.
Eight different observations of six different AGN have been analyzed. No
statistically significant differences from point sources are detected but
significant upper limits of order 20 milliarcseconds are placed on any spatial
extent. Systematic effects limiting the resolution are discussed and some
simple conclusions about the physical size and luminosity densities of the
emitting regions of the AGN implied by the data are given.
