This isn't pestering. This is allowing a bunch of people to learn with you. I am sure that most of the people on the list don't understand these concepts and appreciate Chuck, Brent and others that share their knowledge. -----Original Message----- From: Richard Tenney [mailto:retenney@yahoo.com] Sent: Friday, February 21, 2003 3:01 PM To: utah-astronomy@mailman.xmission.com Subject: Re: [Utah-astronomy] Mirrors My question obviously does belie my complete ignorance of astrophotography and the various methods used to image deep sky objects. When I read that a short focal length scope can photograph a given object X in "half" the time as a longer focal length scope of the same aperture, I see now what they didn't tell me is that the resulting image on the film is also 1/4 the size -- that's a big "duh". So I still fail to see the photographic advantage of a fast scope, unless what is imaged is huge (e.g., the veil nebula) and image size isn't all that important, right? Am I thinking straight here? At any rate, I'll quit pestering the list with my ignorance and go check out a few books on astrophotography and get a clue how this all works. -Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:

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.

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