Since the radius of the Airy disk is 1.22Fk, we can identify Fk as the conversion factor from reduced angle to radius. Thus 4An is the value of the reduced angle at the edge of geometric shadow. In the present case, 8 wavelengths defocusing aberration means the geometric shadow starts at a reduced angle of 4 x 8 = 32, or 26.2 times the radius of the Airy disk.
Eight wavelengths should be barely enough to fit a whole disk between the disk and the interference order centered at a reduced angle of 128. A geometric radius would appear at the center and edge with a diameter in the middle: 32 + 64 + 32 = 128. Of course, the image appears to be somewhat smaller than the geometric radius. A whole disk would drop between the two with a tiny bit of rattle.
Occasionally, ASYMM was asked to calculate an image 10 wavelengths out-of-focus. Some interference from the side order was noticeable in a few cases, but the damage was not severe.
All patterns with a defocusing aberration beyond 10 wavelengths were calculated only with the program APERTURE, which is far less sensitive to sampling error.
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