  Traditional color and airmass corrections can typically achieve ~0.02 mag
precision in photometric observing conditions. A major limiting factor is the
variability in atmospheric throughput, which changes on timescales of less than
a night. We present preliminary results for a system to monitor the throughput
of the atmosphere, which should enable photometric precision when coupled to
more traditional techniques of less than 1% in photometric conditions. The
system, aTmCam, consists of a set of imagers each with a narrow-band filter
that monitors the brightness of suitable standard stars. Each narrowband filter
is selected to monitor a different wavelength region of the atmospheric
transmission, including regions dominated by the precipitable water absorption
and aerosol scattering. We have built a prototype system to test the notion
that an atmospheric model derived from a few color indices measurements can be
an accurate representation of the true atmospheric transmission. We have
measured the atmospheric transmission with both narrowband photometric
measurements and spec- troscopic measurements; we show that the narrowband
imaging approach can predict the changes in the throughput of the atmosphere to
better than ~10% across a broad wavelength range, so as to achieve photometric
precision less than 0.01 mag.
