I'm looking for a purely objective discussion. I'm trying to help a friend understand the difference between telescopes so they can make their own decision, but I want to get it right....

Here's what I understand about my OWN Newtonian (F/5 12.5" primary w/ 2.6" secondary):
1) The secondary mirror blocks approximately 4.3% of the primary mirror.
2) There are 2 mirrors used before the eyepiece, making a very simple system

Here's what I THINK about the Schmidt Cassegrain - please set me straight, someone!
1) There are 4 mirrors involved before the eyepiece, the 2 "secondaries" centered in the middle of the two "primaries".
2) For an equivalent F/5 system (if such a schmidt-cassegrain system exists), would require the second largest mirror to be approximately 9.1 inches in diameter - provided all mirrors are spaced equally apart, plus a folding length of 12 inches. This means that the blockage of the image is 54%!

I have noted that the Newtonian scopes seem acutely brighter than the Schmidt-Cassegrain types of similar aperture. If these calculations are somewhat close, then I surmise that it would require an 18" Schmidt-Cassegrain to gather as much light as a 12.5" dobsonian, provided that the mirrors are INCREDIBLY reflective (98% or more).

This seems a little stiff in my logical mind, so what's the real story, guys?

My 10" SCT has a corrector plate, which is basicly a slightly convex lense, the primary concave mirror and a highly convex seconaray mirror that reflects the light down a tube to the eyepiece. For convenience, a diagonal mirror is placed before the eyepiece to give a better viewing angle for the observer. The secondary mirror is about 3" in diameter which works out to 5.5% of the diameter of the primary.

I don't think that there is as big a diference in the brightness between a SCT and a Newtonian of the same diameter.

I think that where the diference in brightness comes in the play is the diference in the focal length. I know that in a camera, the lower the F ratio the brighter , but smaller the image.

Your scope, and as a lot of Newtonians have a F 5 ratio. My SCT is F10. If I placed a 0.5 Focal Reducer in the light path between my seconadary and eyepiece, It would make it into an F5 scope and increase the amount of light to the eyepiece by 2x. It would also reduce the eyepiece magnification by half.

A given eyepiece at F5 has half the magnification as it does at f10 and the amount of light reaching the eye through the eyepiece would be the same as using, say a 30mm eyepiece in a F10 as it would be to use a 15mm eyepiece in a F5.

Does this make any sence to you?

I've read that a 8" SCT has a limiting stellar magnitude of 13.3 and a Newt with the same 8" apeture is 13.3 as well. I can't see it being much different between the two, even in the larger apetures. What is your friend planning to do, visual observations, imaging or both? Another thing to look at is portability. As most of us know, most of the time, apeture rules... but a big scope is no good if it doesn't get used. If the weight is not an issue, a nice big Dob may be in order, but if the they are wanting to snap a few pics of planetary and DSO targets, I would consider something like Leonard's
set-up. He has a great scope for both visual and imaging.

My two cents,
Larry

Thanks, guys,

Seems I don't yet grasp the design of an SCT. I do now understand (at least one reason) why the image is dimmer: I've always known that magnification decreases bringhtness. Or perhaps more accurately, for a given object, the same amount of overall brightness must remain the same, so if it covers more "real estate", it's intensity must drop so that the total lumens remains the same (except when some of it is lost outside the view).

I need to learn what the corrector lens does. And how does an SCT get F/10 focal length out of 1/4 physical length without more mirrors?!

This link has some easy info. I hope you don't think it too simple.
Mike

http://science.howstuffworks.com/telescope.htm

I like that little link. The little slider, "unveiling" the SCT, really SHEDS LIGHT on the subject!

Come-on Eric, Admit it, inch for inch, My scope is better than your scope..........He, He.......

woah! woah!
put up yer dukes, bucko!