Color-Magnitude Diagrams and Luminosity Functions Down to the Hydrogen-Burning Limit. III. A PreliminaryHubble Space TelescopeStudy of NGC 6791

Using HST ACS/WFC images, we derive the color--magnitude diagram of the old, metal-rich open cluster NGC 6791 to nearly 29th magnitude in V, which is the neighborhood of the hydrogen-burning limit.Comparison with isochrones leads to a discussion of the distance modulus, the reddening, and the age of the cluster. By making a statistical correction for field stars we derive a preliminary luminosity function, and a very tentative mass function. The white dwarf sequence is clearly shown, and has been discussed in a separate paper.