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We present new observations of two Galactic globular clusters, PAL4 andPAL14, using the Wide-Field Channel of the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope (HST), and reanalyze archival data from athird, NGC2419. We matched our photometry of hundreds of stars in these fieldsfrom the ACS images to existing, ground-based photometry of faint sequenceswhich were calibrated on the standard BVRI system of Landolt. These stars aresignificantly fainter than those generally used for HST calibration purposes,and therefore are much better matched to supporting precision photometry of ACSscience targets. We were able to derive more accurate photometrictransformation coefficients for the commonly used ACS broad-band filterscompared to those published by Sirianni, et al. (2005), owing to the use of afactor of several more calibration stars which span a greater range of color.We find that the inferred transformations from each cluster individually do notvary significantly from the average, except for a small offset of thephotometric zeropoint in the F850LP filter. Our results suggest that thepublished prescriptions for the time-dependent correction of CCDcharge-transfer efficiency appear to work very well over the ~3.5 yr intervalthat spans our observations of PAL4 and PAL14 and the archived images ofNGC2419.

Calibration of BVRI Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys