SBC Absolute Flux Calibration

The throughput curves for the imaging modes of the Advanced Camera for Surveys Solar Blind Channel (SBC) have been updated to correct for a 15% - 30% error in the absolute flux calibration. The offset is removed by adjusting throughput curves of various components of the different observing modes, and bringing synthetic photometry into agreement with observed photometry. The resulting curves show that the detector is more sensitive than previously estimated. The practical result of these changes is that the new zeropoints are fainter than before. In other words, until now, the observed astrophysical fluxes of sources have been overestimated. Updated zeropoints for F122M and F165LP have accuracies of 4.5%, while the other filters have accuracies better than 1.7%. New throughput curves and other necessary support files have been delivered to the calibration pipeline so that, from now on, SBC images downloaded from MAST contain the appropriate zeropoints.