Residual at various multipoles (ranging from 1 to 600 and averaged in groups of 40) as a function of the filter step, N. Red curves represent the residual after filtering a data-stream containing only the systematic effect, while blue curves show the residual obtained after filtering the astrophysical signal. Vertical dotted lines represent the tested values and black dashed line is the selected one. Horizontal coloured dotted lines are the mean of all 600 multipoles and the black line represents the sum of the red and blue dotted lines.