GNIRS Sensitivity Estimates

The following table provides preliminary estimated broadband magnitudes reached at 5 sigma (per spectral pixel) in 1 hour of integration. For low background situations, the measurements are assumed to comprise 4 exposures of 15 minutes; for high background situations the lengths of the individual exposures are not relevant. The tabulated values are for a resolution element (2 pixels) and extraction of an optimal window (but not optimal, i.e. weighted, extraction). The observing conditions assumed are 70%-ile image quality, 50%-ile cloud cover (clear), nighttime (80%-ile sky background), 20%-ile water vapor and an airmass of 1.0.

The values given are all for the long slit mode, i.e., with no prisms present and a 2 pixel slit. For observations in cross-dispersed mode, there are reflection losses in the prism as well as absorption losses at the red end of the K window.

Important caveats: These sensitivities are preliminary, derived using the Integration Time Calculator (ITC). Comparison with results obtained during commissioning suggest agreement to within ~15% at 0.9-3.5um and TBD at longer wavelengths. In addition, within each IR band the sensitivity is a very strong and rapid function of wavelength. Applicants should use the ITC to estimate the signal-to-noise ratio at the precise wavelengths of interest and/or over the entire band of interest.

Caution: the atmospheric transmission and emission files currently used by the ITC have a resolution of 1nm and therefore produce telluric features substantially broader than the actual instrumental resolution, especially at R=5900 and 18000. The inter-line signal and noise calculations are correct.

Last update December 10, 2005 Greg Doppmann