PSF at λ = 20 Å of a Wolter-I mirror with the same dimensions as in Fig. 7, plus a sinusoidal perturbation (period 10 mm, amplitude 0.1 μm) on the sole parabola. The result is compared with the PSF of the sole parabolic mirror segment with the same defect, detected in the focal plane of the parabola at a distance of approx. 20 m. The PSF simulation for the Wolter-I mirror with the perturbed parabolic profile returns the same result as the single-reflection case.