Selecting the Optical Train diameter

[allenandholmes]

Please bear with me on this one, as it's one aspect of astronomy equipment that I've always found puzzling. Despite decades of theoretical astro study, consultation of reference materials and persistent forum & web searching including WIKI, I've been unable to find any theoretical "treatment" of what factors determine whether you go for 1.25" or 2"!
Does the answer depend on which OTA you select, everything else (adaptors,filters etc) flowing on from there, or what? How does each size affect eye-relief, focal plane, CCD chip size etc.? Does the selection in any way affect the image quality?
I've been loathe to post these questions; perhaps I need a text entitled "Basic and Fundamental Astronomy for absolute dummies"! "Holmes" just rolls his Kelpie eyes, so he's not going to tell me. :screwy:
Allen

[multiweb]

Well I'm not sure about the theory but for imaging F/5 is a nice base to start from regardless of aperture. I guess 2" will cause less vignetting that 1"1/4 also.

[allenandholmes]

So far I've found the TeleVue site very informative re discussion of optics/eyepieces. Apparently it is worthwhile considering 2" eyepieces (i.e. eyepieces with 2" barrels) for SCT's from 10" and larger because of FOV advantages and the possibility of reducing the number of necessary 2" eyepieces. I'll keep searching and post any further relevent info I find.

[dpastern]

2" eyepieces have a theoritical larger field stop, or that's how I understand it. That allows you to have wider fields of view (TFOV), as well as AFOV. 1.25" simply are impossible to offer with such larger AFOV/TFOV views due to the physics of the barrel size.

Dave