Calibration of the MACHO photometry database

The MACHO Project is a microlensing survey that monitors the brightnesses of ∼60 million stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Galactic bulge. Our database presently contains about 80 billion photometric measurements, a significant fraction of all astronomical photometry. We describe the calibration of MACHO two-color photometry and transformation to the standard Kron-Cousins V and R system. Calibrated MACHO photometry may be properly compared with all other observations on the Kron-Cousins standard system, enhancing the astrophysical value of these data. For ∼9 million stars in the LMC bar, independent photometric measurements of ∼20,000 stars with V ≲ 18 mag in field-overlap regions demonstrate an internal precision σ_V = 0.021, σ_R = 0.019, σ_V-R = 0.028 mag. The accuracy of the zero point in this calibration is estimated to be ±0.035 mag for stars with colors in the range -0.1 mag < V - R < 1.2 mag. A comparison of calibrated MACHO photometry with published photometric sequences and new Hubble Space Telescope observations shows agreement. The current calibration zero-point uncertainty for the remainder of the MACHO photometry database is estimated to be ±0.10 mag in V or R and ±0.04 mag in V - R. We describe the first application of calibrated MACHO data: the construction of a color-magnitude diagram used to calculate our experimental sensitivity for detecting microlensing in the LMC.