7.8 IR Sensitivity

7.8.1 Limiting Magnitudes

Table 7.10 presents the predicted limiting-magnitude performance of the WFC3 IR channel and compares it with that of the camera 3 on NICMOS (NIC3). The calculations are based on an 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. The throughput curves for the WFC3 filters listed in column 2 were used; for NIC3, the most similar wide-band filter was used, and its name is given in column 3.

An online Exposure Time Calculator (ETC) is available (see Section 9.2).

Table 7.10: Limiting-magnitude performance of WFC3 compared with that of the NICMOS NIC3, based on on-orbit sensitivity from SMOV. 

The table provides limiting ABMAGs at a S/N of 10 for the indicated WFC3 filters and for the NICMOS NIC3 with its most similar filters.

7.8.2 Sensitivity

In October 2020, the inverse sensitivities of the WFC3/IR filters were updated based on ~10 years of monitoring observations for five HST CALSPEC standards (WFC3 ISR 2020-10). The data show no evidence of any time-dependence, however the 1-sigma relative photometric repeatability values have been measured at +/- 1.5% (WFC3 ISR 2019-07), making the detection of small sensitivity changes difficult. Scanning mode observations have better repeatability, with a 1-sigma scatter of 0.65% (WFC3 ISR 2021-05).

Independent studies of Omega Centauri in F160W (WFC3 ISR 2020-05) and the HST CALSPEC standards observed with the grisms (Bohlin and Deustua (2019)) show small declines in sensitivity over ~12 years, about 0.1-0.3% per year, while measurements of M35 via spatial scanning over ~5 years show an order of magnitude less effect (about 0.02% per year; WFC3 ISR 2021-05).  Further investigations are underway to better characterize the evolution of the IR detector sensitivity.

Current estimates of the IR photometric uncertainties are ~2% for broad-band filters and 5-10% for narrow-band filters. For a more detailed discussion of WFC3/IR photometric calibration and the associated errors, see Section 7.11 and Section 9.1 of the WFC3 Data Handbook.