A Calibration of NICMOS Camera 2 for Low Count-Rates

TL;DR

This paper establishes a new calibration for NICMOS Camera 2 at low count-rates using extended elliptical galaxies, improving the accuracy of faint source measurements crucial for high-redshift supernova studies.

Contribution

It provides a direct zeropoint calibration for NICMOS 2 at faint count-rates through inter-calibration with WFC3, addressing reciprocity failure issues in low flux regimes.

Findings

Zeropoints of 25.296 (F110W) and 25.803 (F160W) after NCS installation.

Pre-NCS zeropoints of 24.843 (F110W) and 25.498 (F160W).

Calibration results agree with high-count-rate extrapolations.

Abstract

NICMOS 2 observations are crucial for constraining distances to most of the existing sample of z > 1 SNe Ia. Unlike the 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 zeropoint calibration directly applicable to faint sources. This is obtained via inter-calibration of NIC2 F110W/F160W with WFC3 in the low count-rate regime using z ~ 1 elliptical galaxies as tertiary calibrators. These objects have relatively simple near-IR SEDs, uniform colors, and their extended nature gives superior signal-to-noise at the same count rate than would stars. The use of extended objects also allows greater tolerances on PSF profiles. We find ST magnitude zeropoints (after the installation of the NICMOS cooling system, NCS) of 25.296 +- 0.022 for F110W and 25.803 +- 0.023 for F160W, both in agreement with the calibration extrapolated from count-rates 1,000 times larger (25.262 and 25.799). Before the installation of the NCS, we find 24.843 +- 0.025 for F110W and 25.498 +- 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 zeropoints. To avoid human bias, our cross-calibration was "blinded" in that the fitted zeropoint differences were hidden until the analysis was finalized.