--- Richard Tenney <retenney@yahoo.com> wrote:
Sure, that makes perfect sense. But I'm still not quite grokking all of it. Let's say I have two scopes, an f/4 and an f/8, same aperture. My goal is to image, let's say, Saturn, and to have the resulting image of the planet the same size (that is, using the same magnification). How is the image brightness therefore different if the image size is the same (achieved obviously with different eyepieces)?
It's not different. It's identical, given identical apertures. You're confused because you're using eyepiece projection and making the image identical. You get a shorter exposure in an f/4 scope than an f/8 scope of identical aperture because the image is only 1/4 as large at the focal plane. If you make the images of identical size through eyepiece projection, they are of the same brightness because they are now the same size (cover the same area). You are now no longer comparing an f/4 to an f/8. You've made them identical with the eyepiece, as far as brightness goes. C. __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/