12" f5 Reflector on Dobson Mount

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12" f5 Reflector on Dobson Mount

Postby cloudymccloudy » Sat Apr 23, 2016 12:05 pm

Hello SaN forum members, nice to meet you.

I've been interested I astronomy for a while now and previously owned a Meade ETX-125 whilst working in London. I've since relocated back up to the NW - and now have the opportunity of reaching some dark sky sites from home - so looking to buy a new telescope.

My interest is mostly in viewing, rather astrophotography, for Lunar, Planetary and DSO, so looking to buy a good all-rounder scope.

The reflector scope I'm thinking of buying is a: Explore Scientific Ultra Light Dobsonian, Primary Mirror = 305mm, FL = 1525, f5.

My question is: because f5 is considered a fast scope, can I improve its ability for planetary viewing? I've considered using barlow lens, or possibly reducing its aperture to increase its magnification and focal length.

Many thanks.
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Re: 12" f5 Reflector on Dobson Mount

Postby Aratus » Sat Apr 23, 2016 2:10 pm

Its all swing and roundabouts with telescopes. For its size, this is a fairly short tube telescope which can give bright widefield views of the sky. It will be easier to find objects. It will give a good bright view of the planets, but not a very big view.
If you try to get a larger view with a telescope like this you can start to have problems. There is a danger that it won't look focused however hard you try. To stand the best chance you would need a top notch (expensive) eyepiece. Similarly, a high quality barlow. The collimation would have to be spot on too. Even then you might be disappointed with the results.

I think you need to ask yourself how important it is to get sharp, large images of the planets over the bright widefield deep sky objects. If planetary observation is important to you, you shouldn't be going for a Dobson. It was never designed to be a good all-rounder.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: 12" f5 Reflector on Dobson Mount

Postby cloudymccloudy » Sat Apr 23, 2016 8:45 pm

Thanks, Aratus. I feared I was asking too much from one scope. Perhaps I would be better served by an 8" SCT with twice the focal length of a Dobson reflector.
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Re: 12" f5 Reflector on Dobson Mount

Postby Gfamily2 » Sat Apr 23, 2016 10:23 pm

cloudymccloudy wrote:Thanks, Aratus. I feared I was asking too much from one scope. Perhaps I would be better served by an 8" SCT with twice the focal length of a Dobson reflector.

A typical 8" SCT will only have about about 33% longer focal length than your proposed 12" Dob (2000mm compared to 1525mm), so I would reconsider.
'fast' is always relative, because the f/ratio is relative to the aperture; so your 'fast' dob actually has the same focal length as my 'very slow' f/15 102mm Mak, so it will stand the same level of magnification - but with better resolution possible.
Scopes: Meade 8" SCT, Skywatcher 127mm Mak, Raffle winner of SW ST80
For imaging: Pentax K5, Asda webcam, Star Adventurer (new toy)
For companionship: Mid Cheshire Astronomical Group.
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Re: 12" f5 Reflector on Dobson Mount

Postby Aratus » Sun Apr 24, 2016 1:47 pm

I agree with Gfamily2 that with a 12" Dobsonian you are better off than a smaller Dobsonian but you can't get away from the fact that it is still an f/5. An f/5 telescope really doesn't do high magnifications well. Alignment of the optics and quality of the components becomes critical very quickly as the magnification is ramped up. I think planets look pretty good through a Dobsonian, but it isn't the telescope of choice for getting fine detail from them. By all means get a 12" Dobsonisan, (or bigger if you can) but recognise the design is skewed towards deep sky objects, not fine planetary detail.

An 8" SCT is around f/10 and will take higher magnifications much more easily. The central obstruction tends to knock down the contrast when visually observing planets, so it can't be said to be an 'excellant' planetary telescope. (Not a problem when taking photos with it, however) I find it is quite a good choice for planetary detail myself. An SCT also has the advantage of being able to use a focal reducer which then turns the telescope into an f/6.3 with all the advantages that brings. Just remember that this is an 8" and not a 12". That reduction loses quite a bit of light.

There are other possibilities including a standard f/6 Newtonian which are good all-rounders.

If good visual planetary detail is required above everything else then you need to go for a high quality large refractor. They will always have the edge, but the aperture will be even smaller, as will the field of veiw. Not good for faint deep space objects.

Having said all this let me predict that whichever telescope you buy, you will quickly find what it does best, and enjoy it! :)

EDIT: If you haven't already done so there is a basic article on this web site about choosing a telescope. http://www.skyatnightmagazine.com/yourfirstscope
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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