Fourier analysis applied to the polarized standard star BD+59d389 at 5500 Å (200 Å bin). Top: normalized flux differences. The curves trace the partial reconstruction using eight harmonics (solid) and the fourth harmonic only (dotted). The dashed horizontal line is placed at the average of the F values (a₀). Middle: residuals from the reconstruction using the k = 4 harmonic. Bottom: harmonics power spectrum. The dashed line indicates the 5-σ level uncertainty.