Fresnel diffraction with two disks for a central region close to the Arago spot, for Ω = 1.42 m, ω = 1.40524 m, z = 144.3 m, and z − d = 3 m. The top panel is in linear scale; the red dots correspond to Eq. (11) and the continuous curve to the Fresnel diffraction of the disk ω alone attenuated by a factor of 321. Bottom panel: two curves in log10 scales (black, disk Ω alone, red, the system of two disks).