I have a Meade ETX-70 that I've borrowed from my in-laws. They haven't been able to get much use out of it, and so they lent it to me to see what I can do with it. They have 5 different eyepieces: a 6mm Plossl, 4mm Plossl, 25mm Modified Achromatic, 9mm Modified Achromatic, and a 2x Barlow.
Something that occured to me is that all but the 25mm seem useless. If the exit pupil is equal to the focal length of the eyepiece divided by the f-number of the scope (f/5), then the 25mm eyepiece has a 5mm exit pupil, and the rest are worse, with the 4mm having an exit pupil of just 0.8mm. I'm guessing that what this means is that my view through the 4mm is going to be heavily vignetted and basically useless.
However, this seems in contradiction to what I've heard about magnification vs. aperture, namely that I can expect to get 50X per inch of aperture. This scope is 70mm, or about 2.75 inches, and so I should expect to be able to go up to 137X, right? Well, the 4mm eyepiece is just 87.5X, which is well within that range.
Thing is, given the relationship between f-number, aperture, and focal length (namely, that the first is the quotient of the other two), it seems that no matter what the f-number of the scope, it's impossible to get 87.5X out of any scope with a 70mm aperture without the exit pupil being just 0.8mm. So, how does the 50X per inch of aperture rule hold? Is 0.8mm more acceptable than I'm envisioning? I won't actually have a chance to try it out until tonight.
Also, does the barlow lens affect the exit pupil? I wonder if I should try the 25mm with the barlow lens. All I really want to do is split Albireo, LOL.