Haleakalā Sky Polarization: Full-Sky Observations and Modeling

Abstract

Observations of the daytime sky polarization are useful calibration tools for large-aperture telescopes, as well as a testbed for polarized radiative transfer models. We present an instrument capable of measuring the complete full-sky Stokes vector over visible/NIR broad bands. The design utilizes liquid crystal variable retarders and a dual-beam polarization analyzer, allowing for a clear and cloudy sky acquisition cadence near 12 s which shows minimal image artifacts from solar and cloud motion. In this article, we detail the design, full-field calibration methodology, and Haleakalā observations, which provide an absolute polarimetric accuracy on individual Stokes parameters of better than 3% across VRI bandpasses. Angle and degree of polarization images are compared with a single-scattering model and the more complete MODTRAN-4P polarized radiative transfer package. Comparisons with independent measurements atop nearby Mauna Loa show similar polarization structure, but results suggest a relatively larger depolarization from surface reflections beneath Haleakalā.