Relative systematic error as a function of the deviation to the reference mode visibility for the mode height (black line) and linewidth (orange line) for a set of l = 0,1,2 modes with a 2-μHz linewidth, and a signal-to-noise ratio 10 for the l = 0 mode. The continuous lines are the systematic error computed using the method of Toutain et al. (2005). The dashed line is the analytical systematic error on mode height derived assuming there is no systematic error on the linewidth derived from Eq. (B.38). Top left: the deviations to the reference mode visibilities are identical for l = 1 and l = 2. Bottom left: the deviations to the reference mode visibilities are of opposite sign for l = 1 and l = 2. Top right: the deviation to the reference mode visibilities is zero for l = 2. Bottom right: the deviation to the reference mode visibilities is zero for l = 1.