Calibration of the MACHO Photometry Database

Abstract

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 V18 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 magHubble 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.