High Spatial Resolution Hubble Space Telescope NICMOS Observations of Markarian 231

Observations of Markarian 231 at 1.1 mm taken with NICMOS on the Hubble Space Telescope are described. The brightness of the object in the near-infrared, and the inherent short-term stability of the NICMOS optical and instrumental system, enables the application of special observational and analysis techniques that effectively increase high spatial resolution. By these means, we set an upper limit on the size of the core of the active galactic nucleus at 8 mas, corresponding to a radial projected distance of ∼3 pc from the center of Markarian 231.