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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.  |
A general rule is that the gain of a magnitude requires a 2.512X gain in area of light gathering. A loss of a magnitude occurs with the reverse.
In one magnitude brighter skies, you will lose a magnitude on the bottom end. However, the loss of contrast has an effect on the minimum magnification needed to see the same faint magnitude. Because this magnification does not have an effect on a telescope already at the magnification where the maximum resolution is obtained (i.e.where the Airy disc is fully resolved)--you cannot increase the resolution of a scope by increasing its magnification--the scope loses an extra 0.15magnitude in contrast with each magnitude of light pollution. [In plain English, to see the magnitude limit in any scope requires raising the magnification until the maximum resolution of the scope is obtained--somewhere between an exit pupil of 1mm and 0.5mm. That means adding magnification will not allow improved viewing of fainter objects because the Airy disc is now subtending a size where it is getting fainter with increasing magnification. To see fainter requires a larger scope or darker skies.]
So:
1.15 magnitudes loss per magnitude of light pollution. This equals an area gain in light gathering of 2.884 (call it 2.9x) times.
So, if a 10" scope sees to magnitude 15.5 in magnitude 6.5 skies, in magnitude 5.5 skies, it would take a 17" scope to see the exact same contrast limit, and in magnitude 4.5 skies, a 28.9" scope. JUST TO SEE THE SAME THING AT THE RESOLUTION-LIMIT MAGNIFICATION OF THE SCOPE.
I just spent a night observing through a 60" scope in light polluted skies around 2 full magnitudes brighter than my normal dark site.
My 12.5" would have to be 36" to perform the same in those skies as it does at my dark site, so, not surprisingly, I saw more in all deep sky objects through the 60" than I would have in my 12.5" in 2 magnitudes darker skies.
60" is a lot of aperture.
But, my home in LA is two full magnitudes brighter than that! My 12.5" would have to be 104" to equal its performance at my dark site. That isn't going to happen, so I drive to escape the lights.
Make the investment in gasoline to get to a dark site. It will save you money on getting that large scope. Your 8" in a dark site is the equal of 13.6" with one magnitude brighter skies, and 23" in 2 magnitudes brighter skies.
Ouch.
Unless you have a light meter (like the Unihedron Sky Quality Meter), your only gauge of light pollution at your site is to use the various star-counting methods (advocated by the AAVSO, etc.) or finder charts designed to help you pick out the NELM (Naked-Eye Limiting Magnitude), which doesn't give exactly the same result, but close.
The problem with these subjective evaluations of sky darkness is that visual acuity plays a big role in what you can see. Sharper vision will always see fainter stars and give different results. But, your vision should be consistent. If you gauge NELM at a dark site and your usual light-polluted site, the difference is fine to use as indicative of the amount of light pollution present.
Don Pensack