  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.
