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Old 03-01-2015, 08:06 PM
MarcusG
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Location: Perth
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Buying a Second Hand Dobsonain

Hi all!

Longtime lurker on here, only really done some really basic sky imaging, mostly really widefield stuff, as well as some unguided with my Canon 100-400mm lens, which is not exactly ideal (see attached M42 & Saturn images). Anyway, finally getting round to actually getting myself a proper scope, I'm looking at a second hand Dobsonian to start off with (a 10" Skywatcher to be precise), just wanted to know if anyone has any tips as to what sort of things I should look for to make sure all is good with it?

I'll be looking at bolting my Canon SLRs onto it for some imaging as well (mostly planetary given it's not a GOTO), as far as I can tell I'll be needing a T mount and ring to get that setup, I've only seen Celestron branded mounts on my limited searching, will they still sit okay on a SkyWatcher?

Also looking for a 2x Barlow, is there much difference in sharpness/light loss between difference types/brands of Barlow or are they all much of a muchness?

Hope that made at least some sense, look forward to getting some viewing done!
Cheers,
Marcus
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  #2  
Old 04-01-2015, 12:18 PM
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astronobob (Bob)
Casual Cosmos Capturer

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Join Date: Jun 2010
Location: Gold Coast SE QLD
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Hiya Marcus, not bad captures for a lens & i presume, just on a tripod (y)
The thing is with astro photog in not necessarily the GOTO, as this only helps in finding targets. The main need for photos is tracking and guiding, tho lets Not be concerned about Guiding here at the moment, but Tracking should be address if you like to improve somewhat !
Non tracking Dobs are grouse for visual, but virtually any scope would/is rendered pretty useless for imaging without at least tracking - and more useless as the Focal length , lengthens !
The slight star trails in your image of Orion is to be expected for Nontracking, with a lens and an exposure of a few seconds tho with a longer focal length of double of what you used here would increase those trails by twice as much for the same amount of time, ie, if you had said 100-400mm set 'at a guess' 200mm, than at 400mm, those trails be twice as long, plus the image wont be as bright.
So, in short, if you try imageing with a skywatcher 10" with a focal length around 1200mm - 1500mm 'depend on the Focal/Ratio than I wouldnt expect much if anything at all With out Tracking of some soughts !
As far as condition of a purchasing of a scope, theres quite a lot of things to look for -
1st: is it suited for your intensions are, visual or photog, widefield close up, planets or nebulars ?
2nd, The optics condition are important - clean, no marks, scratches, blemishes, mould etc - though a little dust is niether here or there, but with mirrors, be sure to learn how (if need) to clean them, they have soft coatings and can scatch very easily, dont clean them like one would a car windscreen or bath mirror for example !
3rd: Is the base, structure, tube, joints ect all in good nick, firm, no play, or too tight and stiff, may need some attention if you mechanically inclined ?
These are some main ideas for you to consider for an idea anyway !
Hope this has been helpfull.
BTW, do a bit more investigating, reading, research around this Ice In Space forum, plenty of good info from good folk right at your fingertips here !
Also, there maybe an Astronomy Club in your area, do a search there aswell, Clubs are a very good source of info, both theory & hands-on !
Rgrds & all the best with it ,,,
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