  Microarcsecond (uas) astrometry is an indispensable technique to detect
earth-like exoplanets, fully characterize exoplanetary orbits, and measure
their masses --information critical for assessing their habitability. Highly
accurate astrometric measurements can also probe the nature of dark matter, the
early universe, black holes, and neutron stars, thus providing unique data for
new astrophysics. This paper presents technologies of calibrating detectors and
field distortions for achieving narrow field uas relative astrometry with a
focal plane array detector on a 6 m telescope.
