Residual at multipoles ranging from 1 (red) to 500 (blue) as a function of the cut-off frequency f_cut. Vertical dotted lines represent the values of f_cut that have been tested and the black dashed line is the optimal one. The horizontal dotted line represents the mean of the 500 multipoles. In these runs, we have multiplied the amplitude of the periodic systematic effect by a factor 100 to enhance its effect.