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Old 30-03-2016, 10:33 PM
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thegableguy (Chris)
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Limits of an 8" Dob

I took delivery of a new crop-sensor Nikon DSLR today. Chucked on the 1.25" 2x Barlow T-adapter, plugged it into my 8" GSO Dobsonian and got the first photo below, of Jupiter & the four moons. It was the clearest of about 10 shots I took, edited a little to clarify details.

I've taken images of Jupiter before with full-frame cameras (second image below of Venus Jupiter conjunction); though this one was significantly bigger in the viewfinder, it's actually not any clearer.

My expectation is that I've met the resolving limits of either the Dob's optics or the cheap Barlow. Or maybe that's about as good as Jupiter gets with a single exposure.

I was thinking of getting some tube rings & dovetail etc to put this thing on the NEQ6 I just took delivery of, but this photo largely convinced out of it. Its focuser doesn't allow camera use without the Barlow, so if this is about the level of detail I can hope for then it's not worth bothering... is it?

Can someone say definitively that this is about as clear as the GSO 8" f/6 mirror gets? I kinda wouldn't expect it to do better than this for the amazing $500 price tag - those moons are like 10,000km across and about 600 billion km away; to me it's astonishing it can resolve them at all.
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  #2  
Old 30-03-2016, 10:54 PM
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barx1963 (Malcolm)
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That is actually not too bad. If you took a video with your camera (best way is to use Backyard EOS as that will take a video and make it straight into an AVI) then use Registax to stack and wavelet sharpen the resulting images, you may get some sharper more detailed images.

Malcolm
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Old 30-03-2016, 11:07 PM
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thegableguy (Chris)
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Yeah I thought that might be the case - I know that's how you usually do planet imaging.

Just interested to know if the lack of sharpness is down to the optics or the atmosphere. There's clearly a little chromatic aberration here so a coma corrector would help that aspect, but as far as resolution goes I think this scope is probably at its limit yeah? Or maybe not, maybe a big expensive refractor would produce a similar result because the atmosphere is the issue...?
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Old 30-03-2016, 11:27 PM
raymo
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Except on rare nights of incredible seeing, it wouldn't matter much what scope you were using, that's close to what you can get with a single frame.
A $15 webcam from Woolies can produce stunning results in experienced
hands using Registax, or alternatively, use the video mode in your DSLR. Nothing to do with the limits of your 8" Dob.
raymo
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Old 30-03-2016, 11:40 PM
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thegableguy (Chris)
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Okay cool, I'm glad to hear that.

Yeah I know that single frames aren't the way to go for planets etc - totally understand that's a different process and best results come from webcams held to eyepieces - but I was more interested in whether I'd found the limit of what this mirror can do, or if they all butt up against the issue of atmosphere. Sounds like the latter, in which case it might be worth holding on to the Dob for now and perhaps getting dovetail & rings for mounting it.

Or I could just get a secondhand OTA with rings & dovetail already attached for around what the rings & dovetail would cost new from Bintel anyway. There's a 6" f/5 OTA on the classifieds right now for $200. Probably a better idea for a variety of reasons.
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Old 30-03-2016, 11:59 PM
raymo
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I noticed that 6"; it would be a great choice for you.
You would obviously need a coma corrector, but you
could crop off the worst of the coma until you get one.
raymo

Last edited by raymo; 31-03-2016 at 12:02 AM. Reason: more text
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Old 31-03-2016, 08:59 PM
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Geoff45 (Geoff)
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You are nowhere near your limits--video plus good seeing will improve things a lot. This is what I can get with a 4"refractor, so an 8" has to give you something better https://geoffsastro.smugmug.com/Sola...ts/i-tngLmvW/A
Geoff
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