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Originally Posted by norm
Hi All,
If I had a 12"inch dob in say light to polluted skies how would it compare to using an 8"inch dob in absolute dark clear skies (if all things being equal in terms of the same eyepieces being used, same brand of scope, etc).
My thinking is that the 12" would still pick things up easier, but the image would not be as bright/contrasty (if such a word?). On the other hand the 8" in dark skies would show things crisper/bright?
I'm not even sure if there is a point to my question, just something I'm curious about.
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The definative answer: It depends!
Huh? I hear you say!
First off let's look at your assumptions. Assuming both telescopes are identical in terms of optics and thermal equilibrium and focal length (not focal ratio) so that the same eyepiece yields identical magnification. Secondly we need to assume 100% transparent skies with a seeing quality of better than 0.38 arc seconds which is almost never achieved in real life, however for the sake of this arguement that's what's gonna happen!
Crispness, a nice, pretty word, but awfully vague! Loose and turgid thinking at it's worst
Crispness could be the contrast level or it could be the amount of detail visible, or a mixture of both!
Let's define crispness as how sharply defined an image is, how much it "springs out" from the background blackness. If we use this definition we are really discussing the contrast ratio, which is the ratio between the observed background blackness and the object being observed. By using this definition the 8" under dark skies will be usually be the winner over a 12" in an inner city location. Lowering the amount of light pollution will increase the contrast ratio and thus the appparent
crispness of the image.
The contrast ratio becomes most apparent when observing dim surface objects such as extended nebuale (M42) and galaxies like M84 that have low surface brightness which dissapears in to the light polluted background visible at the telescope at light polluted observing sites.
However when observing the likes of the Moon and planets, double stars etc the 12" from a city location will show more detail due to the effect of Dawes Limit. Dawes Limit is the amount of resolving power of a telescope, that is it's ability to distinguish between two pinpoints of light, in otherwords it's ability to show detail. Dawes limit is expressed in arc seconds and the formula is R = 11.6/D where, R = arc seconds, D = primary mirror/lens diameter in cm. Thus an 8" can resolve to 0.57 arc seconds and a 12" to around 0.38 arc seconds.
Brightness: Again a tough call as the apparent brightness of an object can be measured in absolute or relative terms. In terms of absolute brightness the 12" from a city location would be the winner as it collects more light from an object. However, if we factor in contrast to the equation so it becomes a relative brightness measurement then the 8" may well become the winner due to the variablity of light pollution levels.
The bottom line is that it depends on how much light pollution is present.