I'm not sure if I should have put this on the same thread as my star atlas...?

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?

Hi,
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.

Thanks for the info - I may compromise and go with an 8 inch reflector, as portability is definately an issue!

I'd agree 100 % with Dave there, BUT.
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.

Another compromise would be to go for a lightweight SCT, something like a Celestron Nexstar 6se.

I'm afraid that may be a little beyond my reach...

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

If you are concentrating only on planetary and lunar observations then the Skymax is an excellent scope. For deep-sky it won't perform so well as the focal length is very long and the field of view very narrow.

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

Efla, sound advice there from all, if planets are your thing then the skywatcher is a decent scope. But certainly go for as much aperture as you can afford

Keith..

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?

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

The Nexstar was just the first one that came to mind as an example. The Skymax is a sound scope.

www.opticalvision.co.uk/astronomical_tel...rs/explorer-200p_eq5

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

mjc wrote:

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.

Here is an article on Cloudynights about off aperture masking:

www.cloudynights.com/item.php?item_id=548

Paul

If I could bother you good people with one further question (beofre I had over...)

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?

efla wrote:

I know the extra aperture will offer more for fainter mag objects, but would I lose any detail/magnification in planets?

If the optics are bad then yes, otherwise you'll see more in a bigger Dob most of the time.