aTmCam (Atmospheric Transmission Monitoring camera) is a new calibration system to monitor the atmospheric transmission in real time, designed to enable the improved photometric calibration of data acquired by large-area imaging surveys. It will be applied first to the Dark Energy Survey (DES), and hopefully to the Large Synoptic Survey Telescope in the future.

Both the atmosphere and the instruments play an important role in the photometric extinction for ground-based astronomical observation in the optical wavelengths (~300nm-1100nm). 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.

Here, we present a way to get the atmospheric throughput in real time, by measuring the atmosphere throughput and system throughput separately. The latter one could be measured through an end-to-end spectrophotometric calibration (see DECal for more details). The atmosphere extinction is a mixing of Rayleigh scattering from molecules, aerosol and dust scattering from small particles, and molecular absorption (principally by O2, O3, and H2O). The atmospheric transmission spectrum could be easily simulated by some radiative transfer program such as libRadTran ( The two figures below show the relative atmospheric transmission spectrum with different amounts of precipitable water (top) and aerosol optical thickness (bottom).

Image 1

Image 2

This simulation can also be used in reverse. We could measure the amount of each component by knowing the color pattern of the transmission curve. Therefore, we proposed the Atmospheric Transmission Monitoring Camera, or aTmCam, which uses simultaneous measurements of stars with known spectral energy distributions through a set of narrow-band filters. The filters are chosen to allow determination of specific features in the atmospheric transmission spectrum, which then can be used to develop a model that accurately represents the throughput of the atmosphere.

In 2011, we built a concept testing system to demonstrate that brightness measurements of stars at a few wavelengths can be used to derive a model for the transmission of the atmosphere that is as precise as what can be derived with spectroscopic measurements. In 2012 and 2013, we built a aTmCam prototype as a pathfinder and we deployed the prototype at Cerro Tololo Inter-American Observatory (CTIO) for ~40 nights of observing in Oct-Nov 2012 and Sept-Oct 2013 to determined the angular and temporal scale of meaningful changes in the atmosphere. After the success with the aTmCam prototype, we built and deployed a final version of aTmCam at CTIO for Dark Energy Survey (DES) in Aug 2014.

Astronomy Group
4242 TAMU
College Station, TX 77843-4242
Ph: (979) 845 7717
Texas A&M University - College of Science