The ACS Survey of Galactic Globular Clusters. X. New Determinations of
  Centers for 65 Clusters
  We present new measurements of the centers for 65 Milky Way globular
clusters. Centers were determined by fitting ellipses to the density
distribution within the inner $2\arcmin$ of the cluster center, and averaging
the centers of these ellipses. The symmetry of clusters was also analyzed by
comparing cumulative radial distributions on opposite sides of the cluster
across a grid of trial centers. All of the determinations were done with
stellar positions derived from a combination of two single-orbit ACS images of
the core of the cluster in $F606W$ and $F814W$. We find that the
ellipse-fitting method provides remarkable accuracy over a wide range of core
sizes and density distributions, while the symmetry method is difficult to use
on clusters with very large cores, or low density. The symmetry method requires
a larger field, or a very sharply peaked density distribution.
