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 MicronAll Sky Survey (2MASS). The GPC algorithm uses photometry of both primarystandards and moderately bright `tracer' stars in 35 2MASS calibration fields.During the first two years of the Survey, each standard was observed onapproximately 50 nights, with about 900 individual measurements. Based on thephotometry of primary standard stars and secondary tracer stars and under theassumption that the nightly zeropoint drift is linear, GPC ties together allcalibration fields and all survey nights simultaneously, producing a globallyoptimized solution. Calibration solutions for the Northern and Southernhemisphere observatories are found separately, and are tested for globalconsistency based on common fields near the celestial equator. Several results from the GPC are presented, including establishing candidatesecondary standards, monitoring of near-infrared atmospheric extinctioncoefficients, and verification of global validity of the standards. Thesolution gives long-term averages of the atmospheric extinction coefficients,A_J=0.096, A_H=0.026, A_{K_s}=0.066 (North) and A_J=0.092, A_H=0.031,A_{K_s}=0.065 (South), with formal error of 0.001. The residuals show smallseasonal variations, most likely due to changing atmospheric content of watervapor. Extension of the GPC to approximately 100 field stars in each of the 35calibration fields yields a catalog of more than two thousand photometricstandards ranging from 10th to 14th magnitude, with photometry that is globallyconsistent to $\sim 1%$.Comment: 19 pages, 10 figures; Submitted to AJ. The table of secondary standards is available from ftp://nova.astro.umass.edu/pub/nikolaev/ or ftp://anon-ftp.ipac.caltech.edu/pub/2mass/globalcal