Homogeneous photometry. III. A star catalog for the open cluster NGC 6791

We present broadband BVI photometry for the open cluster NGC 6791, based upon analysis of 1764 individual CCD images. We discuss in detail the transformation of the instrumental magnitudes to the standard photometric system of Landolt and then discuss methods for selecting a subset of cluster stars whose photometric indices are most likely to be of high reliability. Color-magnitude and color-color diagrams for the cluster are presented and discussed. A detailed comparison of this photometry with the results of previous observational studies is presented. We make preliminary comparisons with theoretical isochrones provided by D. A. VandenBerg and conclude that an excellent match can be achieved if the metal abundance and age of the cluster are both near the upper end of the range of recent estimates ([Fe/H] approximate to + 0.3, age approximate to 12 Gyr) and the reddening and true distance modulus are both near the low end of the range of recent estimates [E(B-V) approximate to 0.09 mag and (m-M)(0) approximate to 12.79 mag] if the isochrone predictions are reliable. A companion paper will discuss candidate variable stars and stars possibly showing planetary transits. Data tables listing measured magnitudes and standard errors, image-quality indices, a variability index, and equinox J2000.0 equatorial coordinates for 14,342 stars to have been made available to the public through the services of the Canadian Astronomy Data Centre. V similar to 24 Equatorial coordinates only have been provided for a further 1916 stars, the photometry for which we were unable to calibrate because of lack of color information.