SBC Time-Dependent Sensitivity and L-flats

The time-dependent and spatial sensitivities of the SBC detector on ACS were measured using observations of the calibration star cluster NGC6681. The sensitivity of the detector declined by up to 9% since launch, with a rate of 0.5%/year since 2007. New calibration files (IMPHTTAB) were produced, and will be included in the calibration pipeline in order to properly update the photometric zeropoints of every FLT image. The low-frequency L-flats were derived by directly fitting 2D polynomial surfaces to the spatial sensitivity data. The resulting products are smoother than the previous versions, due to the diffrent method of deriving the flats. The corrections in the flats are on the order of 8%. The overall photometric accuracy is 2.5% (except for F165LP which is 3.3%), after combining the low-frequency L-flats with the high-frequency P-flats to make new LP-flats, and applying the new TDS corrections.