Residuals of the observed (red, blue) and the synthetic (black) light curve of images A and D of HE 0435-1223, as obtained by applying the free-knot spline method. The noise used for the synthetic curve is adjusted so that these black residuals show on average (i) the same standard deviation and (ii) the same number of “runs” as those of the observations. As described in the text, the power-law noise has been rescaled to locally match the scatter amplitude of the observed curves, as can be clearly seen in this figure.