Astrometric observation of MACHO gravitational microlensing

Following previous suggestions of other researchers, this paper discusses t he prospects for astrometric observation of MACHO gravitational microlensin g events. We derive the expected astrometric observables for a simple micro lensing event with either a dark or self-luminous lens and demonstrate that accurate astrometry can determine the lens mass, distance, and proper moti on in a very general fashion. In particular, we argue that in limited circu mstances ground-based, narrow-angle differential astrometric techniques are sufficient to measure the lens mass directly and other lens properties (di stance, transverse motion) by applying an independent model for the source distance and motion. We investigate the sensitivity of differential astrome try in determining lens parameters by Monte Carlo methods and derive a quas i-empirical relationship between astrometric accuracy and mass uncertainty.