5.7 UVIS Photometry Errors

The photometric internal precision for the new (2020) time-dependent WFC3 UVIS inverse sensitivities, computed from observations of the four white dwarf standards GD153, GD71, G191B2B, GRW+70 5824, and the G-type star P330E are: ~ 0.5% for the wide-, medium-, and narrow-band filters and ~10 to 15% for the quad filters. However, the absolute error due to systematics and uncertainties in the white dwarf models used as absolute standards can be up to 1.5%.  The signal to noise ratio, per observation, for the standard stars is such that the Poisson error is ~0.2% or less for most 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 error of flat fielding (see Section 5.4), spatial repeatability (~0.5%, but see Section 5.4.2),  readout noise (Section 5.1.2), uncertainty in the encircled energy (~ 0.5% at the infinite aperture - Hartig priv. comm.), charge transfer efficiency corrections (CTE, see Chapter 6) and processing error (see Newberry 1991 and Newberry 1999).