A calibration of NICMOS Camera 2 for low count rates

NICMOS 2 observations are crucial for constraining distances to most of the existing sample of $z>1$ SNe Ia. Unlike conventional calibration programs, these observations involve long exposure times and low count rates. Reciprocity failure is known to exist in HgCdTe devices and a correction for this effect has already been implemented for high and medium count rates. However, observations at faint count rates rely on extrapolations. Here instead, we provide a new zero-point calibration directly applicable to faint sources. This is obtained via inter-calibration of NIC2 F110W/F160W with the Wide Field Camera 3 (WFC3) in the low count-rate regime using $z\sim 1$ elliptical galaxies as tertiary calibrators. These objects have relatively simple near-IR spectral energy distributions, uniform colors, and their extended nature gives a superior signal-to-noise ratio at the same count rate than would stars. The use of extended objects also allows greater tolerances on point-spread function profiles. We find space telescope magnitude zero points (after the installation of the NICMOS cooling system, NCS) of $25.296\pm 0.022$ for F110W and $25.803\pm 0.023$ for F160W, both in agreement with the calibration extrapolated from count rates ≳1000 times larger (25.262 and 25.799). Before the installation of the NCS, we find $24.843\pm 0.025$ for F110W and $25.498\pm 0.021$ for F160W, also in agreement with the high-count-rate calibration (24.815 and 25.470). We also check the standard bandpasses of WFC3 and NICMOS 2 using a range of stars and galaxies at different colors and find mild tension for WFC3, limiting the accuracy of the zero points. To avoid human bias, our cross-calibration was "blinded" in that the fitted zero-point differences were hidden until the analysis was finalized.