Three-dimensional illustration showing PSFs in the two cases of invariant (top panel) and space variant (bottom panel) cases. Vertical axes ( ρ) correspond to positions of points sources in the object at four radial distances from the limb: a = 26.6, b = 48.8, c = 71, and d = 93.3 (in arcsec). The horizontal rectangles show PSF responses in the image plane (a region of η = 43.7 × ξ = 94 arcsec). A non-linear greyscale is used to make the curves more visible.