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(a) Zemax computation of the theoretical field-extent possible with the KPro. (b) Computed vignetting factor as a function of eccentricity in the object visual field. On-axis, the retaining ring acts as an aperture stop, but at a certain eccentricity the last surface becomes an aperture stop. Because both are very close to each other, vignetting always occurs; hence, 100% efficiency is never achieved.
