Paraboloid fit parameters a_P (top), b_P (middle), and θ_P (bottom) of Eq. (14) for the polarization degree of multiple FORS2 fields as a function of target altitude (left), median background polarization degree (middle) and intensity S/N (right). Different filters are color-coded, and filled circles represent our moonlit sky observations. Dashed horizontal lines are the median of the parameters per filter.