Variation of the standard deviation of the Gaussian fit function with V/I amplitude, for the same observational sample that was used in Figs. 3 and 4 (solid lines). The dot-dashed curves represent the standard deviations that would result from the known measurement noise alone. The agreement with the solid curves is evidence that practically all the spread of the points around the regressions in the scatter plots is instrumental, with no discernible solar contribution. The left panel represents the green line pair, the right panel the red line pair.