0 v 2 5 M ay 2 00 3 Spectral Irradiance Calibration in the Infrared . XIV : the Absolute Calibration of 2 MASS

Element-by-element we have combined the optical components in the three 2MASS cameras, and incorporated detector quantum efficiency curves and site-specific atmospheric transmissions, to create three relative spectral response curves (RSRs). We provide the absolute 2MASS attributes associated with " zero magnitude " in the JHK s bands so that these RSRs may be used for synthetic photometry. The RSRs tie 2MASS to the " Cohen-Walker-Witteborn " framework of absolute photometry and stellar spectra for the purpose of using 2MASS data to support the development of absolute cali-brators for IRAC and pairwise cross-calibrators between all three SIRTF instruments. We examine the robustness of these RSRs to changes in water vapor within a night. We compare the observed 2MASS magnitudes of thirty three stars (converted from the precision optical calibrators of Landolt and Carter-Meadows into absolute infrared (IR) calibrators from 1.2–35 µm) with our predictions, thereby deriving 2MASS " zero point offsets " from the ensemble. These offsets are the final ingredients essential to merge 2MASS JHK s data with our other absolutely calibrated bands and stellar spectra, and to support the creation of faint calibration stars for SIRTF.