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Are Deep Sky objects and portability mutually exclusive?


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Hi guys.

I've still not made the choice regarding a scope and don't want to make the purchase until I'm sure.

I'm really interested in the deep sky and wondered what size/type to go for. I know I need a reflector but there is an awful lot of light pollution where I am and the back garden is somewhat boxed in. For this reason I probably want a scope that I can put in the back of the car and get to a dark sky site. (While I'm at it, does anyone know a decent dark sky near the south east of sheffield)?

What is the reasonable size of a reflector to move around?

I'm not into astro photography just yet, so can I use an alt-azi with a decent sized light bucket?

Thanks for any advice in advance.

Cheers.

Ken

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It depends on your ambitions. Using a 70mm refractor, under a very clear dark sky with no moon, in three nights recently, I observed M1, M31, M81 and M82, and Veil, North American, Ring, Dumbbell, Great Orion nebulas as well as various globular and open clusters and Jupiter.

Edited by cs1cjc
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If deep sky is your main interest then an 8" or 10'' dob would be the logical choice. For dso's the more light gathering capabilitie the better. Either will fit across the back seat of your car for transportation to your dark site.

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Have you thought about a MAK or SCT????? They are more compact and living on the fringes of Sheffield you could drive out to the Peaks. I live near Halifax and do have a bit of LP, but just a short drive away and I am out in the sticks. Granted it isn't a fully fledged dark site, but it is better than living in an urban area. I also have a LP filter for good measure.

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If you can reach any location where the milky way is visible to the naked eye then you will get interesting deep-sky views with any instrument from binoculars upwards. An 8" dob is an easily transportable first scope and would give views of hundreds of DSOs. Anything bigger will be considerably heavier.

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If you don't feel (totally) like man-handling a Dobsonian (it's mostly "magnitude penetration" you need), check out VIDEO astronomy. A great way of boosting a (current) portable scope setup, until you "need" a... Scope Leviathan. ;)

Edited by Macavity
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Hi,

Fellow Sheffield resident here... I also haven't got a scope yet.

I've been reading Rod Mollise book on Urban Stargazing, how to beat light pollution.

In it he said it is better to have large aperture and long focal length.

And how you would enjoy more if you don't have to drive everytime, I certainly am liking grab and view currently with my bins. So I am leaning towards Dobs now so I can replicate that with a scope.

Refractor is fine as grab and view, but to get the larger aperture... Too expensive.

Boxed in back yard is not a problem, as the best view is near the zenith (overhead) anyway.

-- Perry Ismangil

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If you don't mind alt-az, and are looking for the maximum aperture for DSO's both near light pollution and in dark skies, it is hard to beat an 8" or 10" Dob reflector. I personally own 90mm & 127mm refractors and a 10" Dob, and though I have seen DSO's in the smaller instruments (and the tracking is nice at higher powers), the 10" is what has allowed me to see much fainter galaxies even from my light polluted front yard.

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My 12 inch Lightbridge dob is easy enough for me to handle (I'm 5'8 and 12 stone, so no giant) and gives great DSO views.

A small refractor is portable, and you will be able to make out some objects from a dark site, but they won't be impressive....in a scope of 10 or 12 inches of aperture objects will really stand out.

Visually, for good views of DSO's, you need aperture...there's no avoiding the fact.

At home I have a superb 6 inch triplet refractor...a fantastic astrophotography scope, and amazing visually on the moon and planets, but on DSO's, visually, the dob beats it hands down.

Rob

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I've been reading Rod Mollise book on Urban Stargazing, how to beat light pollution. In it he said it is better to have large aperture and long focal length.

Large aperture helps. Especially because you can make better use of light pollution filters. Long focal length.... Don't know why he'd say that.

As others have said, and 8" or 10" Dob is perfectly portable (in a car) and there's no reason you should be able to cart it around somewhere dark. There are solutions if you don't have a car (see my signature). When the chips are down, an 8" to 10" Dob is about as portable as an equivalent SCT. Sure, the SCT has a shorter tube but it still needs a tripod and a mount (fork or eq). Either design will fit in a small car and both weigh about the same. In larger apertures, however, Dobsonians become very significantly more portable than SCTs. Don't forget also that Dobs cool much faster than SCTs and will provide a substantially wider field of view. Finally, a Newtonian with good optics is capable of providing very good planetary views. This is something which is often forgotten.

Edited by umadog
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Large aperture helps. Especially because you can make better use of light pollution filters. Long focal length.... Don't know why he'd say that.

Long focal length, he said, means easier to get the magnification required to increase contrast of object by dimming the background sky.

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I lug my 14" dob out to dark sites for the evening occasionally (in my large estate with seats folded down) as well as being my scope of choice for star parties (if I can persuade the wife and kids to stay at home which isn't usually difficult).

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Long focal length, he said, means easier to get the magnification required to increase contrast of object by dimming the background sky.

I don't buy that. It's not the magnification that dims the sky background, it's the diameter of the exit pupil. How much you can magnify before it gets too dim depends on the aperture. Further, different objects have different optimal magnifications (or exit pupil sizes). So you always need a range of eyepiece focal lengths and you can always dim the sky background in any telescope. There are good short focal length eyepieces there are always barlows if you're concerned about it.

Edited by umadog
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I don't buy that. It's not the magnification that dims the sky background, it's the diameter of the exit pupil. How much you can magnify before it gets too dim depends on the aperture. Further, different objects have different optimal magnifications (or exit pupil sizes). So you always need a range of eyepiece focal lengths and you can always dim the sky background in any telescope. There are good short focal length eyepieces there are always barlows if you're concerned about it.

Thanks for explaining this!

I'm sure you are correct, and probably I'm explaining Rod Mollise's position and his experiments badly, so sorry Uncle Rod...

So for now I will retract my 'longer focal length' statement. OP, please ignore me ;)

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Why are you so set on a reflector? A refractor is also good on deep sky objects and is more portable and wont require collmination.

Just me 2 pence worth.

Michael

Agreed. Although not as much bang for your buck, they are very portable and my F5 5" EQ can see plenty of DSOs (inc M51 and NGC 5195 together, M65 and M66 together, M101, M77, Eskimo Neb, Cat Eye Neb to name a few) and there's still more to come when we get into Leo/Virgo/Coma/Canes time.

Having said that, I am contemplating a 10" dob in another year or two. Don't get me wrong I love my frac but a light bucket is a light bucket!

Hang on, i've just started dismantling my own frac argument.

Might be worth getting in touch with an Astro society and try one or two out.

Opinions are good but it's all down to personal preference in the end.

Good luck!

Edited by Double Kick Drum
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How about buying a decent apo frac, a big lightbucket, and SCT, mak....just remortgage the house or something!

Seriously though even using my modest 5" Newt the views are keeping me occupied, admittedly some very fuzzy looking smudges for DSO's but once you get an eye for what you are looking at they kinda jump out at you a bit and my garden is horribley light polluted as is my city sky :/ getting my 3.5" apo doublet up and running (as yet unmounted) but expecting similarly impressive views. Given the cash I would definitely plump for a big dob too, been very tempted by the meade 16" lightbridge but wanna get up close and personal with one before i break the wallet out!

I guess keep your options open you say

I know I need a reflector
but is this statement true? maybe light bucket preferable but the toss up between aperture and portability vs view doesnt make it so black n white. I can only talk from my limited experience with my own equipment but just so you know you can spot DSO from small aperture too ;)

Good luck with your choice and I hope you keep community informed with your purchase, personally I love seeing what setups people use

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I've been reading Rod Mollise book on Urban Stargazing, how to beat light pollution.

In it he said it is better to have large aperture and long focal length.

So if I have big enough aperture I'll be able to see galaxies in daytime? Of course not, which tells us that aperture can't beat light pollution. Larger aperture will always beat smaller aperture under equal viewing conditions, but for DSO viewing, any scope at a dark site will beat any other scope at a bright one. A 4" at a dark site beats a 16" in a city.

Telescopes will show stars in daytime, because stars are effectively point sources and can remain so up to high magnification, while the background sky is darkened. But an extended DSO (galaxay, nebula) is dimmed as it is magnified, just as the sky is, so the contrast remains the same. The threshold of visibility varies with size, which is why we magnify galaxies in order to see them, but if the object doesn't have sufficient contrast against the sky to begin with, then nothing will make it visible to the human eye.

Perhaps the Mollise book is really advising high magnification, especially on point sources (e.g. stars in clusters), in order to make objects more visible when the sky is bright. But it won't beat light pollution. Only thing you can do is get away from it. Observe at night, without the presence of the moon, and as far as possible from man-made light sources. Obvious, really.

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It's my wife's fault I got started with imaging. She bought the DSLR and a T-ring and likes looking at the pictures I take. I shall therefore expect a CCD and filter wheel for my 40th next year.

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But an extended DSO (galaxay, nebula) is dimmed as it is magnified, just as the sky is, so the contrast remains the same.

I don't think it is quite this simplistic. A darker sky background actually helps bring out the faint objects we are looking for, and a larger aperture increases the light grasp of them. A larger aperture gathers more light than a smaller aperture, therefore more magnification can be applied to the same object, and objects appear brighter in the larger scope, therefore effectively increasing contrast compared to the sky background.

That's what makes some of these objects more visible in larger scopes under light polluted conditions.

The exit pupil is key here, which I think is why Rod Mollise mentions a longer focal ratio with a larger aperture. The exit pupil is smaller in longer focal lengths instruments, increasing contrast by causing the background to appear darker, compared to shorter f/l instruments of the same aperture. But the light gathering is increased compared to smaller apertures, so coupled with a darker background (exit pupil) and more light gathering (aperture), one is more likely to see the contrast of the light-emitting object on the (darker) sky background.

Aperture still wins under light polluted conditions, though I agree that more is visible in a smaller scope under dark skies than a large scope under light polluted ones.

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