Graphic representation of ψ_ell defined as the angle between the major axis of the fitted elliptical Gaussian beam and the x-axis of the main beam frame, (XY)_MB, which is aligned with the main beam polarization direction. In the figure the LOS frame is also reported. The angle between the main beam polarization direction and the x-axis of the LOS frame is named ψ_uv and is described in Planck Collaboration II (2014).