Seeing differences ?

[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.

[h0ughy (David)]

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.

well the more light pollution will be suffered in the big scope, but you also get more light of the object your after, as for the 8 performing better in dark skies, well its not physics your trying to defy its common sense. the skies are darker so you will see better, however, the bigger scope will make it easier to identify the object in the city sometimes, Just my experience with the 12" meade and a c8 celestron

[jjjnettie (Jeanette)]

I can see what you're getting at though Norm.
You could get away with using a smaller scope in dark skies and get comparable views to that of a larger scope being used in light polluted skies.
IMO, yes that is true.

Good topic.

[ballaratdragons (Ken)]

Never tried my 12" in Light Pollution, but. . .

I have compared our Clubs GS 8" to my GS 12" using both on the same dark viewing field next to each other. The difference is HUGE! not just slight.

Last Camp, Steve (Janoskiss) abandoned his GS 8" and commandeered my 12" with accompanying "oooh's" and "aaaahhhh's".

Sorry I can't answer for the 12" in light pollution, but Sombrero Galaxy is still bright and stunning with a full Moon! Thanks to Apature!!!

[wavelandscott (Scott)]

I do not know the answer to this question and I am no expert...

Having laid out these "qualifications" my best guess is that aperture will win...

I suspect that the idea that a bigger scope will suffer from more light pollution is a bit of an urban myth but I have no idea how to prove or disprove it.

Yes, dark skies are to be preferred to light polluted ones but the amount of light gathered by a scope is limited by physics...assuming you are not talking "Las Vegas" kind of light pollution but instead "normal" suburb levels of light pollution I think size will win in a 12 inch versus 8 inch battle...

But, what do I know???

Interesting question!

[Dujon]

Too, don't forget that on nights of imperfect seeing (by that I mean atmospheric disturbances abound) then the 8" will be less affected than the 12" as it, literally, looks through a lesser volume (column) of air.

Not having used a 12", nor an 8" for that matter, I cannot offer an opinion on the pros and cons involved.

[janoskiss (Steve H)]

8" under dark skies will easily beat a 12" in moderate light pollution. The difference between these scopes is just under 1 astronomical magnitude (mag). Difference between a dark site and the outskirts of a large city is 1.5-2 mag. So you are still 0.5-1 mag behind at the polluted site even with the bigger scope.

The only exception will be planets and the moon at high powers in good seeing, where the object is so bright that light pollution does not matter at all, and the resolution of the instrument (as limited by the aperture) will be the most important thing.

From personal experience I can tell you that an ED80 (80mm aperture) shows more just outside Marysville 80km from Melbourne CBD than the 8" GS Dob (200mm aperture) from my back yard 20km from the city.

Originally Posted by ballaratdragons

Last Camp, Steve (Janoskiss) abandoned his GS 8" and commandeered my 12" with accompanying "oooh's" and "aaaahhhh's".

Side by side the difference is remarkable although might not be as great as you first think looking at the diff in physcal size between the scope. I did go and buy a 12" Dob shortly after that session with Ken's scope. I do still use the 8" - more often than the 12" actually; but that's because it's easier to lug up and down the stairs and because for observing planets in average seeing the 8" is sufficient.

[ballaratdragons (Ken)]

Originally Posted by ballaratdragons

Never tried my 12" in Light Pollution, but. . .

I have compared our Clubs GS 8" to my GS 12" using both on the same dark viewing field next to each other. The difference is HUGE! not just slight.

Last Camp, Steve (Janoskiss) abandoned his GS 8" and commandeered my 12" with accompanying "oooh's" and "aaaahhhh's".

Sorry I can't answer for the 12" in light pollution, but Sombrero Galaxy is still bright and stunning with a full Moon! Thanks to Apature!!!

I guess that is an unfair statement by me.

As a Deep Sky Observer myself the difference is remarkable towards the GS12".

BUT, on the other hand, if you are a Planetary observer, the 8" blew the 12" away on Planetary stuff. Probably due to the difference in f/ratio. The 12" is f5, whereas the 8" is f6.

Doesn't sound much but it is certainly noticable at the EP, and several of us did the comparison.

[janoskiss (Steve H)]

Originally Posted by ballaratdragons

BUT, on the other hand, if you are a Planetary observer, the 8" blew the 12" away on Planetary stuff. Probably due to the difference in f/ratio. The 12" is f5, whereas the 8" is f6.

I'd have to disagree with Ken on this. As I do own both scopes I can say with absolute certainty that in good seeing the 12" blows away the 8" on planets, even more so than it does on DSOs.

BUT the 12" needs more time to come to thermal equilibrium and before it does the views are disappointing. But once both scopes are properly cooled the amount of detail seen through the 12" is astonishing. E.g., on the surface of Jupiter you see a lot more intricate detail that is not visible in the 8" no matter what the conditions/power/eyepiece. It is something like going from watching VHS tapes on an old tellie to DVD on a high-res display.

I have three 80mm fans on my 12" (will be replaced with a single 120mm soon) and usually it still takes 3-4 times as long to cool down properly (20-30 minutes) than the 8" (5-7 min) with a single 90mm fan.

Other thing to consider is collimation which is more critical with the 12". It goes out more easily and the springs supplied by GSO are way too weak for the weight of the mirror. They sag badly, so you need to either replace them or use the locking screws in conj with the tilt adjust screws in a push-pull arrangement, which is what I do; a little bit of a pain but works for me. If collimation is not spot you will not benefit from the higher resolution of the larger aperture.

[Starkler (Geoff)]

Originally Posted by janoskiss

As I do own both scopes I can say with absolute certainty that in good seeing the 12" blows away the 8" on planets

I witnessed this myself at Snake Valley last year. I observed Mars through 8,10, and 12inch gso dobs with each size increase came an increase in observable detail.

As for the original question, there is no substitute for dark skies for dso viewing. In dark skies I can easily see galaxies and low surface brightness nebula like the helix in my finderscope, where I cannot see them from my backyard with 10 inches of aperture.

[mickoking]

All things being equal the darker the sky the larger the telescope the better? most of the time anyway I saw the companion of Antares easily last night with my 250mm Dob where I struggle to see it with my 300mm. My 120mm refractor just shows the 5th member of the Trapizium while my 300mm Dob shows 6 members with a casual glance and during astronomical twilight. The refractor will show more detail on Jupiter on most nights while my big Dob will show more moons of Saturn. I guess it comes down to experience and observing prefrences with what you will get out out a telescope at a given location.

[ballaratdragons (Ken)]

Originally Posted by janoskiss

I'd have to disagree with Ken on this. As I do own both scopes I can say with absolute certainty that in good seeing the 12" blows away the 8" on planets, even more so than it does on DSOs.

BUT the 12" needs more time to come to thermal equilibrium and before it does the views are disappointing. But once both scopes are properly cooled the amount of detail seen through the 12" is astonishing. E.g., on the surface of Jupiter you see a lot more intricate detail etc. etc.

Thanks Steve.

Very good point!

We will leave the scopes to cool longer next time before comparing. They were only cooled for about half an hour from a warm clubroom to a freezing outside temperature.

[IanW]

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.

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.

[dhumpie]

Aperture always wins regardless of where you observe. The old rule which says that smaller apertures are better for city viewing are all rubbish. From my mag 5.3-5.6 suburban skies, my 10" GS always shows much more detail in all DSO's then both my smaller scopes. In fact for me, the minimum for worthwhile observing is with my 6" from such skies. But I do agree that there is no substitute for a dark sky. A smaller scope under dark skies will always show much more than a large scope under mediocre skies...period.

My 2 cents.

Darren

[casstony]

With regard to DSO's being hidden by light pollution, how much difference does a UHC filter make? How much of the lost contrast (when compared to a dark sky site) is recovered in a 10" or 12" scope?

Tony

[janoskiss (Steve H)]

Originally Posted by casstony

With regard to DSO's being hidden by light pollution, how much difference does a UHC filter make? How much of the lost contrast (when compared to a dark sky site) is recovered in a 10" or 12" scope?

How much? Not much. The UHC filter does let you see some nebulae that you couldn't without it or see them a bit better. But it is not anywhere near the diff between dark and polluted sky.

[ving (David)]

the bigger the scope the better... with my 8" a broadband filter shows more nebula on bright DSOs only. on faint ones it blocks too much light. however on a 12" the amount of light gathered is more so you will see dimmer obfjects with the filter in place...

[Don Pensack]

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

[[1ponders] (Paul)]

Thanks for the details, Don. That "resolves" the issue.

[ColHut (Colin)]

Hmm okay... is there some simple way of establishing easily the actual magnitude limit obtainable with a given apeture given various naked eye visibility levels >

Say
Telescope Ap./ naked eye visual magnitude /max res
6" /4 /10.5

etc.

cheers