
Dob vs Refractor
- efla
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Dob vs Refractor was created by efla
I'm looking around at the moment for a scope and a little confused as I have never used a dob before. Are they useful for planetary obervation? I understand the wider field of view is an issue, but will a 10" dob show the same detail as a 6" refractor for example, or does it all depend on the eyepieces?
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- dave_lillis
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Replied by dave_lillis on topic Re:Dob vs Refractor
Unless you get an absolutely top notch refractor (and at that size you're talking thousands of euros)you will get colour fringes around bright objects like the moon/planets and stars, a dob wont have this problem.
A 10" dob gathers about 3 times the light a 6" refractor will, so objects will be brighter in the 10".
Under good seeing conditions and with equal quality scopes then the 10" wins as a better all round scope, deepsky objects will be more visible and star clusters better resolved.
As for planets, only if the seeing is bad could a 6" refractor possibly beat the 10" for planets, assuming scopes of similar quality, but under most conditions the 10" would be the better scope.
The real drawbacks with the 10" are transportability and collimation, it would a much heaftier and bigger scope to move around. Optical collimation is very important. If it's not done right or is out then your image quality will suffer badly.
If you're in a position to be able to move a big scope around and learn about collimation, then a 10" is the answer for you.
As for eyepieces, most modern eyepeices would be ok for planetary viewing and I wouldnt go to the expense of specialist eyepieces, all the same, bad quality eyepeices will cripple any telescope.
Chairman. Shannonside Astronomy Club (Limerick)
Carrying around my 20" obsession is going to kill me,
but what a way to go.

+ 12"LX200, MK67, Meade2045, 4"refractor
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- efla
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Replied by efla on topic Re:Dob vs Refractor
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- Frank Ryan
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Replied by Frank Ryan on topic Re:Dob vs Refractor
In this hobby,
I've found that if it aint easy to transport and set up,
you will quickly loose interest.
Better to compromise as you say and at least
get some astronomy done rather than get frustrated
and walk away from it.
Shannonside Astronomy Club __________________________________________
Meade ETX-125PE, Bresser 10 x 50 Binos & Me Peepers
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- stevie
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Replied by stevie on topic Re:Dob vs Refractor
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- efla
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Replied by efla on topic Re:Dob vs Refractor
I was looking into this one also (sorry, thanks so much for all the help, I'm conflicted

www.opticalvision.co.uk/astronomical_tel...ins/skymax-127_eq3-2
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- lunartic_old
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Replied by lunartic_old on topic Re:Dob vs Refractor
I think an 8" dob is the way to go, it's not heavy, can be easily transported, offers great views and the price is very reasonable. For deep-sky you won't go far wrong and for planetary work you can build a mask to reduce the aperture to 4" or so to give better views.
Are you close to any astronomy club? If so go along to a meeting or observing seesion and the people there will be only too glad to help and show you views through their scopes.
Whatever you end up buying the best of luck with it and wait for aperture fever to strike.

Paul
Rich Cook
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- Keith g
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Replied by Keith g on topic Re:Dob vs Refractor

Keith..
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- mjc
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Replied by mjc on topic Re:Dob vs Refractor
It may do - I don't know to the contrary. My understanding is that if the mask is off-axis one can increase contrast by avoiding interference from the spider viens - but on the flip-side - resolution goes down with decreasing aperture. Benefits outweigh costs?
BTW efla, I went the 8" DOB route - one can remount it later on an Equatorial mount if you wish to do so later on. Good luck.
Mark
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- stevie
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- efla
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Replied by efla on topic Re:Dob vs Refractor
I believe I may compromise with an 8" reflector....
I'll certainly keep my options open r.e. all of your suggestions whilst I'm shopping around
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- dave_lillis
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Replied by dave_lillis on topic Re:Dob vs Refractor
Why does masking make for better view of planets?
It may do - I don't know to the contrary. My understanding is that if the mask is off-axis one can increase contrast by avoiding interference from the spider viens - but on the flip-side - resolution goes down with decreasing aperture. Benefits outweigh costs?
Hi Mark,
During the times of particularily bad seeing, a scope with a larger aperture can see through multiply cells of air, these cells have differing temperatures and have different refractive indexes (so I read somewhere), this means you'll end up with an inferior image. A scope with a smaller aperture isnt physically wide enoughn to be seeing through 2 or more cells, so it sees through only one cell and could have a better image, this is why masking down a scope can sometimes work, it captures less bad seeing then a bigger aperture scope,
All the same, I have to say that I've rarely seen this work. The bigger scopes generally show more.
When I got the 20", I fully expected the seeing to dramatically reduce the quality of the planetary views, I expected it to be inferior to the 12", you read so many stories on the web. BUT, I'm not exagerating that the 20" has given me some of thee best planetary views I've ever seen, it absolutely floors the 12" every time.
I read somewhere that some of the faith in stepping down big scopes for planetary views came from experiences with big dobs in the 1980s, scopes that were eventually shown to have bad optics (by modern standards) where stepping them down locked out bad areas on the mirrors.
Chairman. Shannonside Astronomy Club (Limerick)
Carrying around my 20" obsession is going to kill me,
but what a way to go.

+ 12"LX200, MK67, Meade2045, 4"refractor
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- lunartic_old
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Replied by lunartic_old on topic Re:Dob vs Refractor
www.cloudynights.com/item.php?item_id=548
Paul
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- efla
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Replied by efla on topic Re:Dob vs Refractor
I'm more or less decided on a 5" cassegrain f/11.81. As I understand it will offer sharp contrast for planetary viewing, and allow a reasonable level of deep sky work.
My question is, are there any reasons to go for either a 10" dob, or 8" reflector? I know the extra aperture will offer more for fainter mag objects, but would I lose any detail/magnification in planets?
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- dave_lillis
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Replied by dave_lillis on topic Re:Dob vs Refractor
If the optics are bad then yes, otherwise you'll see more in a bigger Dob most of the time.I know the extra aperture will offer more for fainter mag objects, but would I lose any detail/magnification in planets?
Chairman. Shannonside Astronomy Club (Limerick)
Carrying around my 20" obsession is going to kill me,
but what a way to go.

+ 12"LX200, MK67, Meade2045, 4"refractor
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