Quote:
Originally Posted by chrisp9au
G'day Richard,
12" dobsonian is F5, 127mm refractor will be F9.4, = better resolution.
12" dob has a large central obstruction, the refractor has none, = better contrast.
At least I think that is what I've been reading!
Can't stop for long, not a cloud in the sky, the scope is out cooling down to a current ambient temperature of 5 degrees.
I might get to see some stars before the moon comes up!
Cheers 
Chris. |
Hi Chris,
You are somewhat confused with a lot of these technical terms, which can easily happen with a lot of the misinformation passed around on the internet. I suggest you buy a book called "Starware edition 4" by Phil Harrington. It is fairly cheap. This is a book based solely on astronomy equipment as opposed to observing. I was one of the contributors and proof readers for this book. Another good book is the Backyard Astronomers Guide by Dickinson and Dyer, which is somewhat more expensive.
Resolution of a Telescope is related solely to its aperture. The F-Ratio has absolutely nothing to do with resolution. The F-Ratio is merely a function of the telescopes aperture and focal length.
There are two traditional methods of calculating the resolving power of a given telescope aperture. The older less commonly used is The Raleigh Criterion and the more common these days is the Dawes Limit. The Dawes Limit is calculated by dividing 4.56 arc secs by the telescope aperture in inches.
For a 5" (127mm) telescope this is .91".
For a 12" (305mm) telescope this is .38".
From the above it is easy to see that a 12" telescope has 2.4 times the resolving power of a 5" telescope. The resolving power of the telescope is not affected at all by the central obstruction. This only affects the contrast and light gathering power. The smaller the central obstruction the less effect it has. Scientific research has shown that if the central obstruction is <20% by diameter, then the Modulation Transfer Function Curves (MTF curves) are essentially unaffected and the images are almost indistinguishable from an unobstructed telescope. In this case the greater resolving power of a 20% obstructed 12" reflector easily outweighs any minor gains in contrast. Some will argue that the smaller scope will cut through poor seeing better than the larger scope but I have never found this to be the case. In those moments when the air settles, the larger scope will easily outperform the smaller scope and you still end up extracting more detail with the larger scope, irrespective of seeing. Further if one wanted to rely on the seeing issue you could always turn the 12" reflector into a 4.5"/F13 APO by using an off axis mask on it.
Cheers,
John B