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 an SNR 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. 

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 IR channel exhibits minor time-dependent sensitivity losses at rates of ~0.1% per year, with greater losses at shorter wavelengths. An upcoming report (Marinelli et al. 2024, in prep) will present new time-dependent corrections for the WFC3/IR detector, which may be manually applied to correct photometry acquired over multiple epochs. The total sensitivity loss since the installation of WFC3 is only ~1-2%, which is less than the photometric uncertainties for wide-band filters. Therefore, users can disregard WFC3/IR time-dependent sensitivity losses when proposing for observations with WFC3/IR since they would not significantly impact the ETC results or orbits requested.

Monitoring of the IR channel sensitivity is carried out via several ongoing calibration programs. Staring mode observations of stellar clusters over a ~13-year baseline suggest a mean sensitivity loss of ~0.13 +/- 0.02% per year (WFC3 ISR 2022-07), while independent studies of the core of Omega Centauri in F160W (WFC3 ISR 2020-05) and of HST CALSPEC standards observed with the grisms (Bohlin and Deustua, 2019) also showed small declines in sensitivity over ~12 years, about 0.1-0.3% per year. Measurements of M35 via spatial scanning over ~5 years initially showed an order of magnitude less effect (about 0.02% per year in F140W; WFC3 ISR 2021-05). After incorporating an additional two years of data, these results now suggest losses of about 0.06% per year in F140W and 0.16% in F098M (Som et al. 2024, in prep).

In contrast to the UVIS detector, staring mode monitoring of HST CALSPEC standards in the IR filters have large systematic uncertainties which limit their 1-sigma photometric repeatability to +/- 1.5% (WFC3 ISR 2019-07). Thus, it has not been feasible to derive sensitivity loss rates directly from these data, despite their significant time baseline (~14 years). However, these data can be used to test the appropriateness of the calculated sensitivity loss rates, such as those previously listed; this approach is fully explored in the forthcoming ISR (Marinelli et al. 2024, in prep). 

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.