Improving Atmospheric Monitoring and Data Quality Analysis Techniques for CMB Measurements from Cerro Toco

The ultimate ground-based cosmic microwave background experiment, CMB-S4, will deploy hundreds of thousands of microwave detectors across multiple telescopes at two sites: the South Pole in Antarctica and from Cerro Toco in Chile. For CMB-S4 to foray into the era of precision cosmology, it is imperative that the environment of these sites is well understood, as they represent a uniquely dynamic foreground component. The amount of water vapor present, for example, impacts atmospheric signal power and transmission of astrophysical signals, which affects measurement sensitivity. Observing conditions can vary significantly year-to-year, impacting observing efficiency and overall survey sensitivity. Furthermore, the presence of ice crystals in tropospheric clouds presents a unique systematics challenge, as these crystals can align and create a polarized foreground.

To better understand these potential impacts, we are deploying additional monitoring equipment to Cerro Toco. This includes an all-sky long wavelength infrared camera which we intend to incorporate into the automated system for data quality flagging for the Simons Array. Such a camera is capable of identifying cloud cover as well as estimating sky temperature. We present the status of this work, including instrument design and early results.