I know I should know this, and am sure Suzy does (
) but can I have a few explainations re why I shouldnt use some EPs in my scope? I have a 10" Dob f/4.7, focal length 1200.
Recently I was going to buy a 40mm EP of Tony (Casstony) and he was very kind to tell me that it wouldnt suit my scope.Thank you Tony.
.....
If the exit pupil from the eyepiece is bigger than your dark adapted pupil, you're effectively reducing the aperture of your scope, with the extra light hitting your eyeball outside of the pupil. For an f/5 scope a 35mm eyepiece is ok from a dark site, 30mm in town, or 35mm plus paracorr in town since the paracorr increases magnification by 15%.
Alex also commented - If only it was a 30mm I would have not hessitated, 
. Just too long for my f/4.5 scope, .
Does this mean that I shouldnt use 40mm and over in my scope, or does it depend on the other measurements - focal length, eye relief, exit pupil etc.? 