Calibration of BVRI Photometry for the Wide Field Channel of the HST
  Advanced Camera for Surveys
  We present new observations of two Galactic globular clusters, PAL4 and
PAL14, using the Wide-Field Channel of the Advanced Camera for Surveys (ACS) on
board the Hubble Space Telescope (HST), and reanalyze archival data from a
third, NGC2419. We matched our photometry of hundreds of stars in these fields
from the ACS images to existing, ground-based photometry of faint sequences
which were calibrated on the standard BVRI system of Landolt. These stars are
significantly fainter than those generally used for HST calibration purposes,
and therefore are much better matched to supporting precision photometry of ACS
science targets. We were able to derive more accurate photometric
transformation coefficients for the commonly used ACS broad-band filters
compared to those published by Sirianni, et al. (2005), owing to the use of a
factor of several more calibration stars which span a greater range of color.
We find that the inferred transformations from each cluster individually do not
vary significantly from the average, except for a small offset of the
photometric zeropoint in the F850LP filter. Our results suggest that the
published prescriptions for the time-dependent correction of CCD
charge-transfer efficiency appear to work very well over the ~3.5 yr interval
that spans our observations of PAL4 and PAL14 and the archived images of
NGC2419.
