**Volume Title** ASP Conference Series, Vol. **Volume Number** **Author** c©**Copyright Year* * Astronomical Society of the Pacific aTmcam: A Simple Atmospheric Transmission Monitoring Camera

Abstract. Traditional color and airmass corrections can typically achieve ∼0.02 mag precision in photometric observing conditions. A major limiting factor is the variabil-ity 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. 1