7.8 IR Sensitivity

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7.8.1 Limiting Magnitudes

Table 7.9 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 7.9: Limiting-magnitude performance of WFC3 compared with that of the NICMOS NIC3, based on on-orbit sensitivity from SMOV. 

Band

Filter


 

Limiting magnitude
in 1 hr


 

Limiting magnitude
in 10 hrs

WFC3

NIC3


 

WFC3

NIC3


 

WFC3

NIC3

J

F110W

F110W


 

27.3

26.1


 

28.6

27.4

H

F160W

F160W


 

26.6

25.9


 

27.9

27.1


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 appears to be losing sensitivity at a rate of approximately 0.13% per year, with no apparent wavelength dependence (WFC3 ISR 2022-07). As summarized in Table 7.10, staring mode observations of clusters show a decrease of 0.1-0.2% per year. Independent studies of Omega Centauri in F160W (WFC3 ISR 2020-05) and the 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. However, 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). For IR staring mode observations, 1-sigma relative photometric repeatability values have been measured at +/- 1.5% (WFC3 ISR 2019-07); scanning mode observations have better repeatability, with a 1-sigma scatter of 0.65% (WFC3 ISR 2021-05).

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.


Table 7.10: Median photometric sensitivity change rates for a number of sampled stars and targets. *Note that the average slope does not include ω Cen core observations.

TargetFilterMedian SlopeMedian Slope (first images)
47 TucF160W-0.12% / year-0.10% / year
M-4F110W-0.19% / year-0.14% / year
M-4F160W-0.14% / yearN/A
ω CenF110W-0.15% / year-0.13% / year
ω CenF160W-0.13% / year-0.16% / year
ω Cen coreF160W-0.20% / year-0.0% / year
Average*Both-0.15% / year-0.13% / year

Median photometric sensitivity losses are tabulated in Table 7.10, which is reproduced from WFC3 ISR 2022-07. In the last column are the sensitivity losses as measured from only the first images of each visit, which are slightly flatter than slopes derived from all images. This is likely because earlier observations were more strongly affected by persistence, since persistence mitigation techniques were not used until later epochs (see Section 7.9.4 for discussion of IR persistence). As a result, the measured flux in many early, non-first images was artificially increased, thus yielding steeper slopes over time when all images are considered. Therefore, the true sensitivity loss is likely closer to the slope derived from the first images.

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