Once we created this catalog containing offending stars, their coordinates, and the size of the scattering, they were matched with guide stars in the previously created guide star files. The physical coordinates recorded from the ACS/WFC images were converted into RA and Dec, then matched to guide star coordinates in the corresponding guide star files with a 5 arcsecond threshold to account for possible alignment offsets. Mismatches are possible, but cases with two very close stars were discarded to mitigate this possibility.

With the guide stars identified by name, we were able to use the information from the guide star catalogs to determine the total ACS filter magnitude for each star. The guide star catalog uses F and J magnitudes. Due to the lack of J magnitude information for many of the stars estimates were made based only on the F magnitude. The photographic F filter has an effective wavelength around 6700Å. Magnitudes in F606W or F814W are similar (to within less than a magnitude). We also normalized the magnitude of each star, based on the exposure time in seconds, to 500 seconds

The resulting mag500 is a measure of the total fluence, or flux multiplied by exposure time:

The greater the fluence, the more charge deposited onto the CCD, and the greater the scattered light artifact for a given star position relative to the ACS/WFC detectors.