I am aware of the idea of looking to maintain an exit pupil to a maximum of 6 to 7mm. Anything larger & we reduce the amount of light entering through the pupil and this in turn will act to effectively reduce the objective's/mirror's diameter.
But how can this be worked out mathematically?
I'll give one of my scope's & EP's as an example to work on:
Alex, I think you just divide the focal length of the EP by the f/ratio of the scope.
A 31T5 in my f/5 scope produces an exit pupil of 6.2mm which is at the limits of what I should be using - much better with a Paracorr, which reduces it to about 5.3mm.
The example you have given is unusual in that you have referred to 65mm EP. I have never come across one of them!
Thanks Rod, but calculating the exit pupil isn't the thing I'm after. By having such a large exit pupil, to what degree am I reducing the diameter of my scope?
This 65mm eyepiece is a Russell Optics XL Super Plossl. Sure it's field of view is only 38* due to the 2" barrel size, but because of this I get no coma problems, & I can comfortably get the maximum FOV a 2" barrel will allow. Not for urban use as the sky glow is a killer. Its eye relief is about 4"! so not to everyone's liking, but it is unique.
Thanks Michael. The effective diameter is smaller than I thought. Not going to stop me using it though. This EP is a perfect match exit pupil wise in the C8. I may get some vignetting, but I'm still to test it out with star fields in the C8. Stunning with the Moon though.
how easy was that! 
