Estimated differential intensities resulting from simulated images in the case of a pupil centrally obstructed (24% of the full diameter). The solid line curves show the best-fit model in the least-squares sense (the model consists of a linear and a cubic component), while the dashed lines only show the linear contribution. The three cases differ in the area used to compute the differential intensity: standard whole area (in black), inner (<2λ/D, in blue), or outer area (>2λ/D, in red). The outer diameter of the Lyot stop is downsized by a factor of 80%, while the central obstruction diameter is set to 35% (1.45 oversizing factor).