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Commanderfish

Biggest useful aperture in Light polluted skies?

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Hi everyone,

What have you found to be the largest useful aperture under moderate light pollution?  I've read that once you go above 8", the light pollution can negate the effects of increased aperture due to the background being too bright.  I'm particularly thinking about DSOs like galaxies here, where large aperture is a big advantage.

 

Fire away!

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I'm not sure there is an upper limit as all additional aperture gives you some gain but there is probably a law of diminishing returns at play which impacts the "step change" in the views that the larger scopes give.

I've stuck to 12" (dob) from my moderately light polluted and obstructed back yard because it's the largest scope that I can easily keep in the house, get out into the garden and move around the garden to find optimum viewing spots. When the seeing is moderate or better the 12" aperture certainly out performs my 4" and 4.7" ED refractors on all targets and has shown me things that I simply could not have seen with smaller scopes.

I've occasionally pondered going up to a 14" dob but, for me, that really would be the limit if the scope was going to get regular use.

 

 

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Large aperture is not really a big advantage for galaxies (or reflection nebulae) compared with dark skies.

However it is an advantage for most other objects which are either point sources (double stars, open and globular clusters), illuminated by the sun (moon/planets), emission nebulae (which  to some extent need to be filtered anywhere) or bright and compact like planetary nebulae.

I use a 12" and occasionally a 16" dob within a few miles of Manchester and it's worth the effort.

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I use 12" SCT, outperforms my 8" Newtonian. I still have to say that galaxies and nebulae are quite hard to see with heavy light pollution, no matter what scope you have. Instead I focus on solar system objects there and go to dark sites for Deep Sky stuff with my 8".

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I've heard that big scopes can cause issues in LP skies... the question is what is big? In the info I've read big means 20"+ aperture with respected big dob owners reporting this. From my experience there were no drawbacks to a 10" under mag 18.5 skies (LP) and only positives. It showed things that just was not possible with smaller scopes. The small footprint and light grasp of the OOUK 12" f5.3 would make it ideal for obs under LP skies, with no drawbacks. IMHO. To really get more light grasp than a 12" scope an 18" or bigger would be needed- when jumping its nice to jump up at least a magnitude of gain.

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I don't think aperture is the critical factor with light pollution.

The focal ratio is what determines how bright or dark the sky appears.

A 'fast' scope will have brighter background than a 'slow' scope.

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6 minutes ago, Kropster said:

I don't think aperture is the critical factor with light pollution.

The focal ratio is what determines how bright or dark the sky appears.

A 'fast' scope will have brighter background than a 'slow' scope.

At the same magnification, the exit pupil will be the same for the same aperture so there will be no difference between the sky background or image brightness between the two.

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2 hours ago, Commanderfish said:

I've read that once you go above 8", the light pollution can negate the effects of increased aperture due to the background being too bright.  I'm particularly thinking about DSOs like galaxies here, where large aperture is a big advantage.

What you read was incorrect. For stars, the bigger the aperture, the fainter you'll see. If the sky is very bright (e.g. daylight) you won't see stars as faint as you would if you used the same telescope under a dark sky: a small refractor may show first magnituide stars in daylight and tenth magnitude at night. But if you've got two telescopes of equal quality and design under the same sky, the larger aperture will always be able to show fainter stars.

With DSOs it's different. To see any extended object there needs to be sufficient contrast between the target and its background. A telescope does not alter this contrast; it makes the target bigger, and alters the apparent surface brightness of target and background. Our threshold of visibility is a function of target size and background brightness, so the alteration made by a telescope can bring an extended object (e.g. galaxy) into visibility. It can only do this if there's sufficient contrast to begin with, and that depends solely on the target and the sky. It is completely independent of whatever telescope you point at it. We can't see galaxies in daytime, and no amount of aperture will show galaxies in daytime.

Light pollution does not hurt some apertures more than others. It hurts DSOs more stars. To see more stars from a light polluted site, choose larger aperture. To see more galaxies, take the scope somewhere darker. A 6" at a truly dark site will easily outperform a 16" at a typical suburban one.

Edited by acey
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I have gone from a C11 to a C8 HD. I live in a town,  & it out performs it ,  because of the light polluted site that I live in.

Love to get a bigger scope again,  but there is no point unless I move. Don't fancy having to drag the scope out some ware. 

 

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50 minutes ago, acey said:

What you read was incorrect. For stars, the bigger the aperture, the fainter you'll see. If the sky is very bright (e.g. daylight) you won't see stars as faint as you would if you used the same telescope under a dark sky: a small refractor may show first magnituide stars in daylight and tenth magnitude at night. But if you've got two telescopes of equal quality and design under the same sky, the larger aperture will always be able to show fainter stars.

With DSOs it's different. To see any extended object there needs to be sufficient contrast between the target and its background. A telescope does not alter this contrast; it makes the target bigger, and alters the apparent surface brightness of target and background. Our threshold of visibility is a function of target size and background brightness, so the alteration made by a telescope can bring an extended object (e.g. galaxy) into visibility. It can only do this if there's sufficient contrast to begin with, and that depends solely on the target and the sky. It is completely independent of whatever telescope you point at it. We can't see galaxies in daytime, and no amount of aperture will show galaxies in daytime.

Light pollution does not hurt some apertures more than others. It hurts DSOs more stars. To see more stars from a light polluted site, choose larger aperture. To see more galaxies, take the scope somewhere darker. A 6" at a truly dark site will easily outperform a 16" at a typical suburban one.

Yep I'm talking about DSOs.  This is bascially what I was getting at in my original post.

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4 minutes ago, Commanderfish said:

Yep I'm talking about DSOs.  This is bascially what I was getting at in my original post.

Sure, so what you read was wrong :). If light pollution is sufficiently bad there is no maximum useful aperture for DSOs because there is no useful aperture.

 

Edited by acey
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A Ten inch Newtonian would work well from my site but a twelve would probably be overkill. However, I'd prefere a longer F ratio over the common short ratio's simply because of better aberration control across the field of view. I'd prefere  a 6" F10 Newt over a 8" F5 despite the narrower field and less light grasp, and a 10" F8 or F10 would be right up my weird little street. :icon_bounce:

Mike

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I had a lovely 10" f6 1/10th wave Orion Optics Dob for a while. Very nice views, easy on collimation and eye pieces and suited to smaller/brighter DSO's, planets and the moon. Scratches head, thinks, why did I sell that one?? ?

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15 minutes ago, Stu said:

I had a lovely 10" f6 1/10th wave Orion Optics Dob for a while. Very nice views, easy on collimation and eye pieces and suited to smaller/brighter DSO's, planets and the moon. Scratches head, thinks, why did I sell that one?? ?

im sure you will get another one stu :icon_biggrin:

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51 minutes ago, Stu said:

I had a lovely 10" f6 1/10th wave Orion Optics Dob for a while. Very nice views, easy on collimation and eye pieces and suited to smaller/brighter DSO's, planets and the moon. Scratches head, thinks, why did I sell that one?? ?

Portable and easy to set up, too.

thass' why I never sell my scopes...  :D

Could have easily sold my 12" when I upgraded to 16" but it has become the 'plug and play' scope because it is so easy to transport, etc. If I could acquire a 16 or 20 incher @ f3.0, I would be pretty chuffed!

 

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If a smaller aperture will encourage you to take it to a dark site then a smaller apeture will win...

:Dlly

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When we drive for weekend to a dark site, we always have a car completely stuffed full already. I found out that a 10" Dob simple wouldn't fit in the car at all. And the dark site is the place where I wanted to use my bigger scope. Therefore my choice fell on an 8" SCT. An alternative would have been a suitcase Dob, but that requires a lot of setting up, which didn't attract me. Although that film above wasn't too discouraging.

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FWIW, I found that a larger aperture's better resolution surprised me on globular clusters - I didn't expect them to change so much between 5" and 10", and there are a fair number that are visible even in my light polluted skies, so that might still be a positive to larger apertures still. 

However, galaxies are mostly rubbish in town - it's better to drive somewhere dark. My 5" scope somewhere dark beats my 10" in town. 

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1 minute ago, AndyWB said:

FWIW, I found that a larger aperture's better resolution surprised me on globular clusters - I didn't expect them to change so much between 5" and 10", and there are a fair number that are visible even in my light polluted skies, so that might still be a positive to larger apertures still. 

However, galaxies are mostly rubbish in town - it's better to drive somewhere dark. My 5" scope somewhere dark beats my 10" in town. 

I think that's definitely true Andy, the resolution benefits really do show on globs, they look amazing with a larger scope.

My 66mm refractor on safari in Tanzania gave me views which rivaled an 8" back home!!

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1 hour ago, Linda said:

When we drive for weekend to a dark site, we always have a car completely stuffed full already. I found out that a 10" Dob simple wouldn't fit in the car at all. And the dark site is the place where I wanted to use my bigger scope. Therefore my choice fell on an 8" SCT. An alternative would have been a suitcase Dob, but that requires a lot of setting up, which didn't attract me. Although that film above wasn't too discouraging.

you wont believe how quick it takes to set up a truss dob...

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