Asymmetric beams do not destroy information. Upper panel: for a multipole moment of ℓ = 500, we show a slice through the covariance matrix for the reference simulation with symmetric beam. Middle panel: the relative difference between the covariance matrices calculated with symmetric beam (F ^{− 1   Ref}) and strongly asymmetric beam (F ^{− 1   Asym}) is only of the order 1%. Lower panel: beam profiles used in the comparison.