  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.
