Photometric Calibration of the Acs Ccd Cameras

The absolute flux calibration of the standard WFC and HRC filters is derived from the available constraining observations of spectrophotometric standard stars. Values for the encircled energy (EE) of one arcsec radius relative to an infinite aperture radius are derived for hot stars and compared to the EE for cooler stars. The sensitivity degradation for the five year ACS lifetime is defined and used to correct the ACS photometry before deriving revised quantum efficiency (QE) curves for the CCD detectors. F850LP observations of red stars with WFC are made consistent with F850LP photometry for hotter stars by decreasing the WFC QE curve longward of 9400Å. Broad band QE changes with a maximum of 2.3% for WFC are also included in the revised QE curves for both CCD cameras. Revisions of the average filter transmissions of up to 4% are required to bring both broad and narrow band photometry into exact agreement with synthetic photometry from the primary white dwarf (WD) stars. The remaining inconsistencies between WD and cooler star photometry are discussed but not resolved because of the limitations of the present data set. Until these discrepancies are resolved, the transfer of the fundamental WD absolute flux calibration to cooler types of F8-G0 should be regarded as uncertain by at least 2% in the 7000-10000Å range. HST flux calibrations for spectral types cooler than G may have even larger errors.