Same as Fig. 10 but reducing the nominal Pluto surface pressure (P_surf = 12.2 μbar) to 6 μbar (upper panel), 3 μbar (middle panel), and 1 μbar (lower panel). All the cases displayed here correspond to the tenuous atmosphere regime. The height of the central flash, (R₀/r₀)² (Eq. (32)), exceeds the limits of the frame in the first two panels and is visible in the lower panel. As P_surf approaches zero, the flash tends to the Poisson spot with a height equal to unity, as illustrated in Fig. 3.