Principle of a stellar occultation by the atmosphere of an opaque spherical body with radius R₀, replaced here by a disk perpendicular to the figure. Upper panel : tenuous atmosphere case, where the rays grazing the surface of the occulter at R₀ cannot converge toward the shadow center, creating a dark shadow of radius r₀ (thick black line in the observer’s plane) with a Poisson spot at its center in O′. Lower panel : dense atmosphere case, where there is a layer with radius R_CF that focuses the rays toward the shadow center, creating a central flash in O′. In the gray zone of radius r′₀, an observer at P receives a ray coming from the primary (secondary) image plotted as the red (blue) dashed line.