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David,
I have a 12.5" f/5 scope with similar dimensions. I
use a 14" diameter "tube", a 2" low profile focuser (about 1-3/4" above inside
surface of tube), and about 1/2" -3/4" back focus. A 2.14" diagonal gives me
about 0.3" fully illuminated field. The 2.6" about 0.8" fully illuminated field.
Either one of these will fully illuminate the field at 100X or above. This is
also where I do most of my viewing. Below this magnification, the intensity will
drop off toward the edge of the field. Of course, the intensity will drop off
less with the larger diagonal. However, even at 50X (the lowest I would
recommend with a 12.5" f/5) the intensity at the edge with my 2.14" is only
about 70%. Some would say that you can't see the difference between 70% vs. the
85% edge illumination with the 2.60". In practice, I have both diagonals for my
dob. Can I see the difference? If I could compare them side by side, I might.
But the differences in edge illumination and planetary performance aren't large
enough to for me to see between sessions when I swap diagonals. I leave the 2.6"
in mostly out of personal biases. I do more deep sky work than planetary
viewing. But the planetary views with the 2.6" are very impressive. Also, the
diagonal offset isn't as critical with the larger diagonal.
With regard to your numbers, I would not go with a
design where the center 0.25" isn't fully illuminated.
Albert
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