Star (orange disk) located behind an object of radius R₀, as seen by an observer at P (Fig. 2). The central flash layer of radius R_CF is shown in green. The star appears as a disk with radius r_* projected at the body distance, its center being at distance r from the projected body center. Upper panel : case r > r_*, the atmosphere produces two images, a primary (secondary) image sketched as shaded the region delimited by the red (blue) line. In a given direction defined by the angle θ, the two points A and B along the stellar limb have points A’ and B’ as images, respectively. Lower panel : case r < r_* the two stellar images merge and form an Einstein-Chwolson ring. The apparent stellar radius r_* and the atmospheric scale height H have been greatly exaggerated for better visibility. In real cases, the stellar images are much more compressed.