The tabular data presented here was derived using on-orbit data that was processed with ground-based flat field reference files. (See WFC3 ISR 2009-30 and
WFC3 ISR 2009-31.) These flat fields are in error by up to several percent on large spatial scales. More accurate zeropoints (see Online arrow below) have been determined by using better flat fields (see Sections
5.4.3 and
5.7.4) and by deriving filter-specific solutions instead of making a low-order fit across wavelengths. Monitoring of photometric performance over the first year of operations has shown the WFC3/UVIS detector to be stable to rms ~ 0.5% (
WFC3 ISR 2010-14) and the WFC3/IR detector to be stable to rms ~ 0.5% to 1.0%, depending on filter (
WFC3 ISR 2011-08).
Throughputs are presented in graphical form as a function of wavelength for each of the imaging filters and grisms in Appendix A:WFC3 Filter Throughputs. Given the source characteristics and the sensitivity of the WFC3 configuration, calculating the expected count rate over a given number of pixels is straightforward. The additional information required is the encircled energy fraction (ε
f) in the peak pixel, the plate scale, and (for the spectroscopic modes) the dispersions of the grisms.
The sensitivity information is summarized in Tables 9.1 and
9.2. In these two tables, and in the following discussion, the filter transmission functions are denoted
T(
λ), and the overall system response function (apart from the filter transmission) is denoted
Q(
λ). The terms “counts” and “count rates” always refer to the number of detected
electrons, which is converted to data numbers, or DNs, upon readout according to the gain factors for the detectors. The measured gain is 1.55 e
−/DN for the UVIS channel and ~2.4 e
−/DN for the IR channel (see
Table 5.1).
In Tables 9.1 and
9.2, the following quantities are listed:
