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mark81

Light gathering Vs light pollution

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Hi all,

So, a few weeks back a friend came to visit and had a look through my ST80. He was impressed. Today he has told me he is thinking of getting the same scope and that got me thinking... I'm in quite a rural location with bortle 3/4 skies and he is in Greater London, which I guess is more like bortle 7.  So how much of a difference will there be in what can be seen from his location compared to mine?  To see objects with similar results will he need more aperture and if so, how much more? Maybe the answer goes down to people's own opinion/experience's but any help would be appreciated...

Thanks

Mark

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Reduced light pollution makes a lot of difference when observing deep sky objects. Hardly difference any when observing the Moon, planets and double stars.

It's difficult to be precise but my guess is that he would need something like 150mm get anything like the views your 80mm will show of deep sky objects at your location :icon_scratch:

Of course when he comes to your place, his 150 will outshine your 80mm by quite a margin !

Edit: then you will want to upgrade, then he will and so on and so on !

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Hi . when it comes to planetary and lunar observing, light pollution does not make much difference in my experience. You can get good results on lunar and planetary viewing from light polluted areas.

But when it comes to DSO , the faint fuzzies then light pollution can be a nightmare on these targets. To such a point, locating or viewing faint DSO can be near impossible in heavy light polluted areas. To give you an example. My 350mm aperture scope in light polluted areas will easily be outgunned by my 200mm scope at a dark site when it comes to faint fuzzies. All the extra aperture and light gathering ability of the 350mm in light polluted areas is no competition when it comes to the  200mm aperture scope at a true dark site location on faint DSO. 

 

Hope this helps

 

 

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I started with a 76mm in big city light pollution & even the easy dso's, (open clusters) were very faint. Doubles (and a few planetaries) were OK as said.

Much better now with an 8" newt but only the brightest Globulars are interesting and almost all galaxies are just invisible.

I don't think any amount of aperture truly compensates for the lack of dark skies, but the bigger the better.

I would say an 8" scope is the minimum for decent visual observing of dso's in Bortle7/8.

Edited by lenscap
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Huge difference in observing from Bortle 3/4 and Bortle 7/8.

I'll give you my experience.

Bortle 3/4 100mm ST102 scope could easily show at least 4 larger galaxies from Markarian's chain.

Bortle 7/8 8" Dob will struggle to show M51 (just two cores and you have idea that something might be there), while M81/M82 could be seen most of the time but very faint. As a contrast same scope at Bortle 3/4 and same M81/M82 targets - I was under impression I was looking at car headlights :D. M51 showed spiral structure and the bridge and looked almost like on images.

So even 8" in heavy LP won't be able to render anything close to 80mm under dark skies.

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I've owned a 12" Newtonian and where i live i can't even see M1 (as an example) because the light pollution is so poor.

At our dark site i've seen M1 in 15x50 binoculars.

But only a few weeks ago from home i could see Saturns shadow cast upon its rings in the same light polluted back yard with my refractor.

Swings and roundabouts.

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I think the best thing your friend can bring to urban observing is a good positive attitude and at least an 8" Dob :D. If he starts observing DSOs from Greater London, he ought to know that it's going to be tricky, but any failures shouldn't be upsetting. Your friend could also work through something like the Urban Observing Program. Many of the objects will look fine in an 8" Dob and all can be found with a little effort. And the best thing about all the work he does in the city is that when you two get together at your darker site, after his jaw is picked from the ground of a star filled sky, he'll be better equipped to savour it and drink in its delights :biggrin:

Edited by Rob Sellent
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i had a 16" in a white zone and a 80mmf/5 refractor showed 1000x better in a green zone, sometimes u cant overcome the LP by size

joejaguar

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I recall some years ago spending many nights trying to see M33 with my (now departed) 8" SCT in my suburban skies.  Not a chance, I concluded I was looking straight through it.  I was lucky enough to use the same scope under Bortle 2/3 in a very rural location in Devon and was amazed at the brightness and level of detail I could see in the same target.

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Regardless of scope type or size, you need something well baffled with a black tube.
That is the best way to deal with the off axis stray light.

As an experiment, to simulate light pollution, shine a red light torch across the end of any scope, just grazing the lens on a refractor, or the corrector plate on a compound scope. Or just illuminating a bit of tube in an open newt.

Look through the eyepiece, and the sky background is red. Just how bright red depends on the scope.

Considering a newt. If it has not-very-black paint, then the scattered light is well reflected. If it is really black or flocked, there is little light scatter.
Clearly a long FL with dew shield helps. The same applies to a refractor.

The difference between a newt and a refractor (or compound scope) is the ease of improving the blackening.
Flocking or painting a newt is easy. But do you want to dismantle a complex scope?

A more expensive scope tends to have better paint and baffling. A side by side of similar refractors will show, and the better contrast from better lens coatings.

I once did a side by side of an ST102 and WO80 at 3x the price. A world of difference in contrast. Under polluted sky the smaller WO80 would have shown more.

I used to own an Orion Optics UK 8" newt. Excellent glass. It gave super views. But the tube inside was best described as dark grey. Under polluted sky contrast (or lack of) made it like a cheap scope.

Hope this doesn't create confusion. David.

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