Q–U plane for moonlit sky polarization in R band. Each point corresponds to a box of 30 × 30 pixels and the y position with respect to the center is colored in different shades of blue. The median value of Q and U is shown as an orange cross, while the value at the optical axis (x₀ = 1226, y₀ = 1025 pix) of the instrument is shown in cyan. Dashed lines indicate Q = 0. Typical errors on Q and U are on the order of 10⁻⁵ (see Table B.1).