FLUX CALIBRATION OF THE ACS CCD CAMERAS I. CTE CORRECTION

The flux calibration of HST instruments is normally specified after removal of artifacts such as a decline in charge transfer efficiency (CTE) for CCD detectors and optical throughput degradation. This ISR deals with ACS/WFC CTE losses, which had been considered negligible for bright stars prior to the demise of the ACS CCD channels on 2007 Jan. 27. Following the revival of ACS WFC during the Servicing Mission 4 (SM4) in 2009 May, CTE corrections are now typically several tenths of a percent and should be included, even for our bright standard star observations that utilize a standard reference point which is only 512 rows from the CCD amplifier B readout corner. For such bright standard stars with negligible background signal, a simple correction algorithm with an accuracy of better than 0.1% is derived, which eliminates the need to execute the CTE correction code for the complete image.