Inhomogeneity in the pupil of the analytical structure function of the fitting residuals D_ϵ for two different relative distances normalised by . Within each panel, on the left is D_ϵ(x + Δ/2, x) (SF₁), and on right is D_ϵ(x + (Δ/2, 0)^T, x) (SF₂). The dots emphasise the actuator positions. Panel a: 2D local piston. Panel b: 2D pyramid. Panel c: 2D axisymmetric Gaussian profile. Panel d: 2D axisymmetric ALPAO profile. Panel e: 2D sinc function. Panel ſ: binary mask analytical model. – Aperture: disc of diameter 95%D.