Quote:
Originally Posted by rogerg
Well, I still don't think that's the full story... would it not be more correct to say: "you want more light in get more aperture (for the given focal length)" ?
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Well of course the visible FOV recorded on your sensor real estate for a given image scale will be a function of aperture and FL. If you modify one independently from the other you'll modify your FOV. But aperture wins still for a given FOV, which really is what we're talking about when we're taking a picture of a DSO. We frame it and we make sure everything "fits in".
Let's take a concrete example.
Scope 1: 5" newtonian F/5 - 650mm FL
Scope 2: 11" SCT F/1.8 - 504mm FL (give or take depending on mirror position at focus)
Both these system will give a similar image scale between 2.8 and 2.9asp respectively. Scope 2 resolving power is better because of larger aperture and Scope 2 light gathering power is also better because of larger aperture.
At the end of the day I'm still covering 2.9 x 1.8 degrees of sky regardless with both scopes at a very similar image scale. So from the CCD perpective the same photons are coming in but loads more of them for the same unit of time. Now you'll say yeah but if I make the C11 slower to F/5 to match the newt then the image will be darker. Yes but I'm only looking at maybe only 1 x 0.8 degrees of sky?