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Hi Rod & All,
Yep, the maths look good to me and so far as I understand it, that has been the established lore since I was a boy (and beyond).
If the 'scope is producing an 8mm diameter beam then there is no way it can fit into a into a 6mm hole (your iris) no matter how you do the maths. Similarly, you can't fit a 5/8ths bolt into a 1/2" nut -- it just won't go.
You may ask then why do the manufacturers make eyepieces like the 41Pan?
Of course in a "slow" system, for example an f/10 C-11, the magnification is x68 and the exit pupil for that scope/eyepiece is not much over 4mm -- very comfortable.
Put that 41mm Pan into your 'scope (which has a fairly similar fl to the C-11) and you get a pretty similar magnification of x61, but the exit pupil jumps to a massive 8.2mm and some of the light gathered by the 'scope will be effectively lost as there is no way you can get all of it inside your eye as it emerges from the eyepiece.
I have seen instances of people (no names, no pack-drill -- over 10 years ago) using a 10" f/4.5 with a 50mm plossl at x25. Fantastically wide field -- 2 degrees but, the exit pupil was 10mm!
In your case as I said, the 6.1mm exit-pupil the 31mm T5 produces in the 20" f/5 probably won't quite "exceed the limit" -- but it is pushing close to the boundary that your average 30-something oberver's dilation can achieve. I don't (can't) dispute that both yourself and John might well be able to dialate to 6.5mm (or even further), but if you can, you will be a bit above average certainly.
I'm 46 yo, and I proceed on the assumption that 6mm (or fractionally over that) is my limit. The 31mm T5 in my 'scope would have produced nearly 6.4mm of exit pupil. That is the main reason I didn't buy it. The 26mm T5 produces a smalller field certainly (57 arc-mins -vs- 68 arc-mins) but the exit pupil is 5.4mm -vs- 6.4mm.
The 28mm UWAN (had I known of it, I'd have bought it) would have been the best comprimise of all (for me) -- x79, 63 arc-mins of field and a 5.7mm exit pupil -- and much cheaper!
Best,
Les D
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