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might be able to use simple geometry to find the filter size at which vignetting begins.
back of the envelope for ~f4 or slower comes out as:
required filter diameter = d/fNo + diag
where d is the distance between the filter and the chip, and diag is the chip diagonal.
In practice, some vignettting will be OK, so you could use slightly smaller filters than calculated without much penalty. I am using 1.25 filters with smallest possible spacing on an f4 system and the 1600 is illuminated just well enough (some vignetting, but still OK).
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