Effective area of an obstructed mirror shell with R₀ = 148.5 mm, f = 10 m, α₀ = 0.212 deg, L = L^∗ = 300 mm, and the X-ray source at infinite distance, 6 arcmin off-axis. The curves are computed for three different values of . The reflective shell has a Pt/C graded multilayer, whose d-spacing variation in depth is a power law, d(k) = a   (b + k) ^− c, where k = 1 ... 200, a = 115.5 Å, b = 0.9, c = 0.27. We moreover assumed a Pt thickness-to-d-spacing ratio Γ = 0.35, a roughness rms σ = 4 Å, and a stack that ends with Pt on top.