A Global Photometric Analysis of 2MASS Calibration Data

We present results from the application of a global photometric calibration (GPC) procedure to calibration data from the first 2 years of the Two Micron All Sky Survey (2MASS). The GPC algorithm uses photometry of both primary standards and moderately bright "tracer" stars in 35 2MASS calibration fields. During the first 2 years of the survey, each standard was observed on approximately 50 nights, with about 900 individual measurements. Based on the photometry of primary standard stars and secondary tracer stars and under the assumption that the nightly zero-point drift is linear, the GPC ties together all calibration fields and all survey nights simultaneously, producing a globally optimized solution. Calibration solutions for the Northern and Southern Hemisphere observatories are found separately and are tested for global consistency based on common fields near the celestial equator. Several results from the GPC are presented, including establishing candidate secondary standards, monitoring of near-infrared atmospheric extinction coefficients, and verification of the global validity of the standards. The solution gives long-term averages of the atmospheric extinction coefficients (in magnitudes per air mass), AJ = 0.096, AH = 0.026, A = 0.066 (north) and AJ = 0.092, AH = 0.031, A = 0.065 (south), with formal error of 0.001. The residuals show small seasonal variations, most likely due to changing atmospheric content of water vapor. Extension of the GPC to ∼100 field stars in each of the 35 calibration fields yields a catalog of more than 2000 photometric standards ranging from the 10th to 14th magnitude, with photometry that is globally consistent to ∼1%.