Pushing the Limits of Ground-Based Photometric Precision -- Sub-Millimagnitude Time-Series Photometry of the Open Cluster NGC 6791

We present the results from a three night, time-series study of the open cluster NGC 6791 using the Megacam wide-field mosaic CCD camera on the 6.5m MMT telescope. The aim of this study was to demonstrate the ability to obtain very high precision photometry for a large number of stars. We achieved better than 1% precision for more than 8000 stars with 14.3 < R < 20.1 and sub-millimagnitude (as low as 0.36 mmag) precision for over 300 stars with 14.6 < R < 16.3 in the field of this cluster. We also discovered 8 new variable stars, including a possible delta-Scuti variable with an amplitude of 2%, 6 likely W UMa contact binaries, and a possible RS CVn star, and we identified 5 suspected low-amplitude variables, including one star with an amplitude of 3 mmag. We comment on the implications of this study for a ground-based survey for transiting planets as small as Neptune.