If the pupil could be described in both amplitude and phase by a figure of revolution, diffraction patterns were calculated using the self-contained program APERTURE. This program allows the user to specify both the transmission and the aberration as a function of radius. After selecting a value of the defocusing aberration, APERTURE calculates the point spread function (PSF) and the encircled energy. Once the PSF has been calculated far enough away from the central diffraction disk, the modulation transfer function can be generated.
APERTURE is relatively fast. If the radius is divided into 300 sampled points, it can deliver the 200 calculations comprising a typical in-focus PSF in less than a minute using a 25 MHz 386/7. It then shows the image in shades of gray on a VGA screen.
At this point another program, POSTDR, can be invoked. POSTDR translates the intensity file into a 128 x 128 halftone image using the PostScript™ page-description language. POSTDR allows one to adjust the contrast and brightness of the picture to fit within the limited dynamic range of printing paper.
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