Perfecting the Photometric Calibration of the ACS CCD Cameras

Abstract

Newly acquired data and improved data reduction algorithms mandate a fresh look at the absolute flux calibration of the CCD cameras on the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The goals are to achieve a 1\% accuracy and to make this calibration more accessible to the HST guest investigator. Absolute fluxes from the CALSPEC\footnote{http://www.stsci.edu/hst/observatory/crds/calspec.html} database for three primary hot 30,000--60,000K WDs define the sensitivity calibrations for the WFC and HRC filters. The external uncertainty for the absolute flux is $\sim$1\%, while the internal consistency of the sensitivities in the broadband ACS filters is $\sim$0.3\% among the three primary WD flux standards. For stars as cool as K type, the agreement with the CALSPEC standards is within 1\% at the WFC1-1K subarray position, which achieves the 1\% precision goal for the first time. After making a small adjustment to the filter bandpass for F814W, the 1\% precision goal is achieved over the full F814W WFC field of view for stars of K type and hotter. New encircled energies and absolute sensitivities replace the seminal results of Sirianni et al. that were published in 2005. After implementing the throughput updates, synthetic predictions of the WFC and HRC count rates for the average of the three primary WD standard stars agree with the observations to 0.1\%.