Quantities used in the wave optics calculations. A star at infinity at the left of the figure along the OO′ direction sends a plane wave through the occulter atmosphere, considered as a thin phase screen XOY. A source point S in this screen, with polar coordinates (R, θ), emits an elementary wave toward the observer in the shadow plane at P, with an inclination angle χ with respect to the normal of the screen. If the atmosphere is spherical, the point P can be placed without loss of generality along the O′x axis, at a distance r = |x| from the shadow center. The phase of the elementary wave received at P depends on both the phase shift induced by the atmosphere at S and by the distance l traveled by the wave between S and P.