Moonlit sky polarization patterns from Cerro Paranal

We investigated the polarization patterns from the moonlit sky as observed from the European Southern Observatory at Cerro Paranal. The moonlit sky background can be significant in astronomical observations and thus be a source of contamination in polarimetric studies. Based on sky observations during full Moon with FORS2 in imaging polarimetric mode, we measured the polarization degree and intensity at different wavelengths and scattering angles from the Moon, and we compared them to theoretical and phenomenological single- and multiple-scattering models. Single-scattering Rayleigh models are able to reproduce the wavelength dependence of the polarization as long as strong depolarization factors that increase with wavelength are introduced. Intensity data, however, require the inclusion of single Mie scattering from larger aerosol particles. The best models that simultaneously fit polarization and intensity data are a combination of the two single-scattering processes, Rayleigh and Mie, plus an unpolarized multiple-scattering component. Both Mie and multiple scattering become more dominant at longer wavelengths. Other factors such as cloud depolarization and the sunlight contribution at twilight were also investigated. The present study underscores the importance of accounting for moonlight scattering to enhance the accuracy of polarimetric observations of astronomical targets.