A low-cost, all-sky LWIR imager for cloud monitoring at the Simons Array

An all-sky long-wavelength infrared camera system was designed for automated monitoring of clouds at the remote Simons Array site in the Atacama desert of Chile. High, thin ice clouds are not always obvious from existing weather information, including optical imagery, and these clouds can create spurious polarization signals which interfere with mapping the polarization of the cosmic microwave background (Takakura et. al 2018). A commercial FLIR Lepton module (4800 pixels, sensitive to 8-14 μm) controlled by a Raspberry Pi is combined with a reflective sphere to create an efficacious and cost-effective cloud imager. A Raspberry Pi Camera Module simultaneously takes optical images for cross-referencing data. The prototype includes a compact welded steel support to position the sensors above an aluminum sphere, and custom 3D-printed housing for weather protection. This results in views of the sky above 30 degrees in all directions. The camera control software is written using open-source Python libraries. Along with qualitative information about cloud cover, the LWIR sensor data can be used for radiometric measurements of approximate cloud and sky temperature. The prototype has been tested in the Albuquerque area to optimize the design and ensure reliable operation, before remote operations in Chile. Data has been taken in a variety of conditions, and preliminary data analysis is underway.