The estimated errors for the chip-dependent inverse sensitivity for the WFC3 UVIS channel computed from observation of the three white dwarf standards are: Error in the mean: 1.3% (statistical) and Absolute error: 1.3% (systematic, uncertainties in the white dwarf models used as absolute standards). The signal to noise ratio, per observation, for the standard stars is such that the Poisson error is ~0.2% or less for the wide band filters. For those users who need precise photometry, a complete error analysis would include Poisson noise of the science target as well the additional sources of photometric error of flat fielding (see
), spatial repeatability (~0.5%, but see
), temporal variation (see
), readnoise (
), uncertainty in the encircled energy (~ 0.5% at the infinite aperture - Hartig priv. comm.), charge transfer efficiency corrections (CTE, see
) and processing error (see Newberry, 1991 and Newberry, 1999).