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Looks like this thread is almost done but may I add that when SDM telescopes rebuilt "Zamar" ( 20" F5) part of the brief was to be able to image with it. Peter Read did the Calculations and came up with a 3.5" secondary from the original 3.1. Technically there would be a difference but as Les has stated you will never notice . I've given this much though and you would need 2 identical scopes sided by side and flick between them . It will never happen.
On the flip side you would think that the larger secondary could reduce what is visible at the extremely faint end however I confirmed seeing with AV at this years QLD Astrofest PGC 2184533 when stalking a few bucket list Globs (G352 & G351) of Andromeda . At +17.57 I would have thought not possible so I don't think the slightly larger secondary will make any difference at all. (Observing report to follow)!!
Cheers W.
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