The product of the reflectivities of the two segments of a Wolter-I shell with α₀ = 0.212 deg, at 30 keV. The reflective surface is a multilayer coating, a Pt/C graded stack with 200 couples of layers. The d-spacing in the multilayer follows the power-law model d(k) = a   (b + k) ^− c (Joensen et al. 1995), with a = 77.4 Å, b =  −0.94, c = 0.223, a Pt thickness ratio Γ = 0.42, and a 4 Å roughness rms. The solid line is directly computed from the multilayer structure, whilst the dots are the result of the inverse computation from the effective area in Fig. 3.