Calibration of the MACHO photometry database

The MACHO Project is a microlensing survey that monitors the brightnesses o f similar to 60 million stars in the Large Magellanic Cloud (LMC), Small Ma gellanic Cloud, and Galactic bulge. Our database presently contains about 8 0 billion photometric measurements, a significant fraction of all astronomi cal photometry. We describe the calibration of MACHO two-color photometry a nd transformation to the standard Kron-Cousins V and R system. Calibrated M ACHO photometry may be properly compared with all other observations on the Kron-Cousins standard system, enhancing the astrophysical value of these d ata. For similar to 9 million stars in the LMC bar, independent photometric measurements of similar to 20,000 stars with V < 18 mag in field-overlap r egions demonstrate an internal precision sigma(V) = 0.021, sigma(R) = 0.019 , sigma(V-R) = 0.028 mag. The accuracy of the zero point in this calibratio n 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 publ ished photometric sequences and new Hubble Space Telescope observations sho ws agreement. The current calibration zero-point uncertainty for the remain der 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.