All-sky polarization measurements of the total solar eclipse on 21 August 2017

On 21 August 2017 we measured skylight polarization during a total solar eclipse in Rexburg, Idaho, using two all-sky polarimetric imagers. The all-sky polarization images were recorded using three simultaneously operating digital singlelens-reflex (DSLR) cameras with good low-light sensitivity. Each camera was equipped with a 180 degrees field-of-view fisheye lens to view the entire sky and each lens contained a fixed linear polarizer orientated at 0 degrees, 60 degrees, and 120 degrees, respectively, to recover the first three Stokes parameters. Skylight polarization was measured from sunrise to sunset in the cameras' blue, green, and red channels. Before and after totality, the maximum sky polarization occurred in its usual pattern with a band of maximum polarization positioned 90 degrees from the sun. However, during totality skylight polarization became nominally symmetric about the zenith. This was observed clearly in the blue and green channels and less obviously in the red channel, which had a greatly diminished signal. At and near the observation site, we also operated an infrared cloud imager, a hand-held spectrometer to measure surface reflectance, and an AERONET solar radiometer to characterize the atmospheric aerosols. This ancillary data set provided a complete characterization of the conditions of the surrounding atmosphere and underlying surfaces.