Post-SM4 ACS/WFC L-Flats and Photometric Errors from Observations of Stellar Fields

We present the first comprehensive post-SM4 study of the low-frequency correction to the ACS/WFC flat-field (L-flat). Using a large sample of constant brightness sources from archival image data, we map out variations in their brightness from multiple dithers over the WFC CCD area, and fit a low-order flat-field model that minimizes those variations. We find strong similarity between our resulting model and the current database L-flat, with <~ 1% differences everywhere, indicating that the L-flat has remained quite stable over time. However, even after correcting for the flat-field, we find that the photometric scatter of dithered point sources is larger than expected from current error models, ranging between 0.5% to 3% depending on instrumental magnitude and exposure time. This suggests that one or more of the terms in the current error model, which includes contributions from the various reference files (e.g. flats, darks, biases) and CTE losses, are underestimated. The team is now considering revising the error arrays and expanding the scope of the photometric error analysis.