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 σV = 0.021, σR = 0.019, σV - 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