Computed aperture diffraction PSF at λ = 3000 Å, source at infinity, of a grazing-incidence parabolic mirror with f = 10 m, a minimum radius R₀ = 150 mm, and a length L₁ = 300 mm, resulting in a circular corona aperture of 2.25 mm width. The dashed line is the usual diffraction pattern of a straight slit of equal width, while the accurate computation (solid line) is obtained by computing the exact diffraction pattern integrated over circular coronae. A high-frequency modulation in the latter would also be superimposed owing to the diffraction of the 2R₀ diameter circular aperture, but in real cases is canceled out by the finite resolution of the detector.