--- Chuck Hards <chuckhards@yahoo.com> wrote:
Aperture alone determines flux density (througput). Focal length determines magnification- and now you just invoke the inverse-square law for brightness. Twice the focal length, twice the mag., 1/4 the brightness.
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)? (Sorry to tax the patience of the rest of you that do have a clue with my density here...) -Rich __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/