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° field-of-view fisheye lens to view the entire sky and each lens contained a fixed linear polarizer orientated at 0° , 60° , and 120° , 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° 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.
Multispectral space telescopes with visible to long wave infrared spectral bands provide difficult alignment challenges. The visible channels require precision in alignment and stability to provide good image quality in short wavelengths. This is most often accomplished by choosing materials with near zero thermal expansion glass or ceramic mirrors metered with carbon fiber reinforced polymer (CFRP) that are designed to have a matching thermal expansion. The IR channels are less sensitive to alignment but they often require cryogenic cooling for improved sensitivity with the reduced radiometric background. Finding efficient solutions to this difficult problem of maintaining good visible image quality at cryogenic temperatures has been explored with the building and testing of a telescope simulator. The telescope simulator is an onaxis ZERODUR® mirror, CFRP metered set of optics. Testing has been completed to accurately measure telescope optical element alignment and mirror figure changes in a cryogenic space simulated environment. Measured alignment error and mirror figure error test results are reported with a discussion of their impact on system optical performance.