Hi Morton,
I think I understand what you were trying to know.......
Check out this article by Nagler talking about this subject;
http://skytonight.com/equipment/basics/3077091.html
To explain:
The size of the shadow cast by the secondary into the beam of light emerging from the eyepiece will be the result of its size in proportion to the primary.
So using your example of the 10" F5 with say a 42mm eyepiece.
Exit pupil is, as you say; 8.4mm.
If the diagonal was a 54mm, then this represents approx 21% of the primary on diameter.
21% of 8.4mm = ~1.76mm size shadow
If we assume your pupil size at ~6mm then the shadow is about 29% (on diameter) of the beam of light entering your eye.
Taking an extreme example to make the point:
Make the eyepiece a 60mm in your 10"F5 Dob.
Exit pupil = 12mm
21% of 12mm= ~2.5mm shadow in the centre of the light beam.
About 42% of the beam going into your eye.
I suspect from the above that you could easily go to 50mm on the 10" F5 without the shadow being that noticeble. Different story during daytime where you would pick up the shadow quite clearly.
A bigger problem with the very low powers will be the sky "whiting out" on you! The magnification is so low that the scope picks up all the backlight in the sky and you lose contrast.
Many will say that going above 7mm wastes aperture because the light is falling outside your pupil but ignores the fact that you get the wide field of view you were looking for......... a problem on the bigger Dobs.
:-) Clear as mud?
Yours,
Greg Reilly