6.8 UVIS Sensitivity

6.8.1 Limiting Magnitudes

Table 6.10 presents the predicted limiting-magnitude performance of WFC3. The calculations are based on optimal extraction of a point source. The limiting ABMAG at a S/N of 10 was calculated for a 1-hour and a 10-hour exposure.

Table 6.10: Limiting-magnitude performance of WFC3 based on on-orbit sensitivity from SMOV4.
The table provides limiting ABMAGs at a S/N of 10 for the indicated WFC3 filters.

Band

Filter

Limiting magnitude in 1 hr

Limiting magnitude in 10 hrs

WFC3

WFC3

WFC3

NUV

F225W

26.7

28.1

U

F336W

27.3

28.7

B

F438W

27.3

28.7

V

F606W

27.9

29.2

I

F814W

27.1

28.4


Chapter 9 gives further details on estimation of exposure times. Current estimates require use of the WFC3 Exposure Time Calculator (ETC), available online.

6.8.2 Sensitivity

The two WFC3/UVIS CCDs have very different quantum efficiencies in the UV, where UVIS2 is nearly 30% more sensitive at ~2200 Å. At wavelengths longer than ~4500 Å, the sensitivity of the two chips is within about ~1%, as shown in Figure 5.2. An independent photometric calibration is applied for the two UVIS CCDs, and temporal changes in the inverse sensitivity are accounted for in the latest UVIS photometric calibration (WFC3 ISR 2021-04). These new solutions correct the image header 'inverse sensitivity' keyword (PHOTFLAM) to account for changes of ~ 0.1 - 0.2% per year according to filter. Figure 6.21 shows an example of the time-dependent behavior of UVIS photometry over about 12 years, in this case for UVIS2 and the F814W filter. It also demonstrates the improved repeatability in observations acquired with spatial scans compared with that of staring mode observations.

The updated UVIS solutions improve the chip-sensitivity ratio (PHTRATIO) in FLT/FLC data by up to 1%, in agreement with early dithered star cluster and standard white dwarf observations. Accounting for the time-dependence also improves the encircled energy (EE) correction by ~ 1% in the ultraviolet filters and by ~ 0.5% at wavelengths larger than 7500 Å, in close agreement with the 2009 EE values. The updated calibration makes use of improvements in the HST CALSPEC models as well as an increase in the Vega reference flux. 

For the latest information about the UVIS photometric calibration and the inverse sensitivity tables, users may visit the WFC3 Photometry webpage. The accuracy of the inverse sensitivity values is ~ 2% on average, with ~ 5-10% errors for narrowband filters. Additional discussion is provided in the Section 9.1 of the WFC3 Data Handbook.

Figure 6.21: UVIS Photometry in F814W

Percent change in the F814W observed count rates versus observation date (in MJD) for photometry of five CALSPEC standards measured in a 10-pixel radius aperture.  Monitoring observations of four white dwarfs GRW+70d5824 (filled circle), GD153(diamond), GD71 (square), G191B2B (triangle), and the G-type star P330E (horizontal triangle) are shown for two UVIS2 subarrays, where black points indicate drizzled (drc) photometry for amp C and red points for amp D. For comparison, photometry in the corresponding single calibrated (flc) images is shown in grey. Photometry based on scanned data is overplotted for GRW+70d5824 (blue) and GD153 (purple). The solid line shows the fit to the photometry for all five standards, for both staring and scanned images, and indicates a loss in sensitivity of 0.11% per year +/- 0.07% in this filter.