Stellar Cluster Fiducial Sequences with the Advanced Camera for Surveys
  We present color-magnitude diagrams of five Galactic globular clusters and one Galactic open cluster spanning a wide range of metallicity (-2.1 < [Fe/H] < +0.3), as observed in the F606W (broad V) and F814W (I) bands with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. These clusters are part of two large ACS programs measuring the star formation history in the Andromeda halo, tidal stream, and outer disk. In these programs, the clusters serve as empirical isochrones and as calibrators for the transformation of theoretical isochrones to the ACS bandpasses. To make these data more accessible to the community, for each cluster, we provide a ridge line tracing the stars on the main sequence, subgiant branch, and red giant branch, plus the locus of stars on the horizontal branch. In addition, we provide the transformation of the Victoria-Regina isochrones to the ACS bandpasses. 
Thomas M. Brown, Henry C. Ferguson, Ed Smith (STScI), Puragra   Guhathakurta (UCO/Lick), Randy A. Kimble, Allen V. Sweigart ...