HST Proper Motions of Stars within Globular Clusters
  The stable environment of space makes HST an excellent astrometric tool. Its
diffraction-limited resolution allows it to distinguish and measure positions
and fluxes for stars all the way to the center of most globular clusters. Apart
from small changes due to breathing, its PSFs and geometric distortion have
been extremely stable over its 20-year lifetime. There are now over 20 globular
clusters for which there exist two or more well-separated epochs in the
archive, spanning up to 10+ years. Our photometric and astrometric techniques
have allowed us to measure tens of thousands of stars per cluster within one
arcmin from the center, with typical proper-motion errors of ~0.02 mas/yr,
which translates to ~0.8 km/s for a typical cluster. These high-quality
measurements can be used to detect the possible presence of a central
intermediate-mass black hole, and put constraints on its mass. In addition,
they will provide a direct measurement of the cluster anisotropy and
equipartition. We present preliminary results from this project, and discuss
them in the context of what is already known from other techniques
