ACS Flat Fields and Low-order “L-flat” Corrections from Observations of 47 Tucanae

The uniformity of the WFC and HRC detector response has been assessed using multiple dithered pointings of 47 Tucanae. By placing the same stars over different portions of the detector and measuring relative changes in brightness, low frequency spatial variations in the response of each detector have been measured. The original WFC and HRC laboratory flat fields produce photometric errors of 6 to 18 percent from corner-to-corner. The required low-order correction (L-flat) has been applied to the lab flats, and new flat fields have been delivered for use in the calibration pipeline. Initial results indicate that the photometric response for a given star is now the same to ∼1% for any position in the field of view. A comparison of the improved flat fields with high signal observations of the bright earth at 3300 Å and with preliminary skyflats at 7700 Å also shows agreement to within ∼1%.