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Is there a way to visually, or through bench testing of the scope (with normally available tools, like a Cheshire and circular grid target (easily made) to confirm full illumination (visability) of the primary field of view?
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Visually I couldn't tell how mirror spacing affects field illumination but if you stick a CCD at the back, shortening the distance between primary and secondary will increase vignetting. Having said that if the spacing primary/secondary is shorter than recommended you will see increased spherical aberration in your in/out of focus airy discs. You lose the symmetry and some rings get brighter so the light is no longer focused at the same point.