Per cent difference between the window function obtained using Level-S (see Sect. 5.2) and the FEBeCoP window function (see Sect. 5.1), at 70 GHz. The left panel is an enlargement of the right panel, concentrating on the low multipoles. The contribution from the main beam (mb), near sidelobes (nsl), and far sidelobes (fsl) is shown. The agreement between the Level-S window function computed using the main beams and the FEBeCoP window function is evident, as presented also in Planck Collaboration IV (2014). The effect, 0.1–0.2%, of near and far sidelobes is clearly visible at low ℓ. At high ℓ the difference is mainly simulation variance, due to the small number (49) of CMB realizations.