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250pds vs 80ed pro for DSO imaging


Mikyg

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Hi all, I'm after a bit of help for my dads observatory. He already has the 250pds with the skywatcher reducer/coma corrector but we can't seem to get any crisp images. if the exposures are more than 20 seconds we get trails. He has it on a NEQ6 Pro with the 250pds, on it, a Sky-watcher ST80, atik 314+, qhy5, atik filter wheel and the autofocuser. 

 

He was thinking of getting rid of the 250pds and getting the evostar 80ed pro but we have been told that the 5stop is faster on the 250pds.

Could anyone explain which they think would be better for dso imaging and list why? We're by no means professionals so would really appreciate any help. Thanks

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Hi Miky!

I'm just finally getting everything where I'd hoped a bit over 2 years ago. I got into this madness for DSO imaging. And my peculiar target is Nebula. So I knew pretty much what I was after, then found my way there.

As I researched and my equipment was chosen, I found myself at a Triple Apochromatic ED80, for the image quality I was after. My reasoning evolved to what I wanted was a telescopic camera lens in the end. My choice of camera was a poor one initally, and it held me back until a month ago when a friend offered me to use his Atik Infinity. And if I wanted, to sell me said same. His offer and price was irresistible to me. So generous was he. It took my kit from crummy, to wow, in the first image.

Many, many, images have been taken with smaller scopes and high quality lenses. So the combination is what needs to be found to get satisfactory images. But you appear to be headed the right way now. I prefer a refractor myself for all the reasons they make great imaging telescopes. But the Triple APO's are a tad pricey. Don't be fooled about the fStop of the telescopes. Mine is a 6. But does a fabulous image with the right camera. 

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I'm not particularly experienced but in your thread you mention star trails. Start trails might be caused from an unstable mount due to wind (250mm reflector is a wind sail) vibration of you/dad walking around, balance or poor polar alignment. Have you looked at those possible causes already?

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7 hours ago, happy-kat said:

I'm not particularly experienced but in your thread you mention star trails. Start trails might be caused from an unstable mount due to wind (250mm reflector is a wind sail) vibration of you/dad walking around, balance or poor polar alignment. Have you looked at those possible causes already?

Hi yes, ive looked at all these things. I believe ive got the polar alignment set quite well now as its pretty good at finding most things and it doesn't really get much wind as its in the observatory and also we don't walk anywhere near the scope as its all ran from the warm room via the computer. I have aslo given him my hiking gps unit to use for ascom setup so unless im missing something really big ive got a feeling it could be the weight of the scope.

 

below is an image of the scope when we was just testing the pier out so its not fully loaded like it is now

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On 16/8/2017 at 00:13, Mikyg said:

more than 20 seconds we get trails

Hi. Your setup suggests you're guiding the mount. If it's doing only 20 seconds then I'd look at the software. If that checks out OK, then I'd say flexure is a candidate. Replace the bottom dovetail with a wider longer plate, lose the adjustable rings on the st80 and secure it with its original -non alignable- rings. With your big pixel camera, I'd say it's guidable . HTH and good luck.

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On 8/17/2017 at 16:04, alacant said:

Hi. Your setup suggests you're guiding the mount. If it's doing only 20 seconds then I'd look at the software. If that checks out OK, then I'd say flexure is a candidate. Replace the bottom dovetail with a wider longer plate, lose the adjustable rings on the st80 and secure it with its original -non alignable- rings. With your big pixel camera, I'd say it's guidable . HTH and good luck.

Thanks for that, we don't have the original rings for the st80 as we just got the ota on its own, would be nice to get some thought.

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Are you running PHD2 and if so have you ran the drift alignment tool? I normally allow it to run for about 10 mins in both directions and do this twice. As @alacant suggested ditch the st80 rings you have for more suitable solid rings and bolt it down to your main scope.

spill.

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30 minutes ago, spillage said:

Are you running PHD2 and if so have you ran the drift alignment tool? I normally allow it to run for about 10 mins in both directions and do this twice. As @alacant suggested ditch the st80 rings you have for more suitable solid rings and bolt it down to your main scope.

spill.

I am running it on phd2 but haven't ran the drift alignment tool, will definitely give it a try though thanks. Sadly he doesn't have the original rings with the st80 but will be keeping an eye out for some.  Thanks

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is your scope balanced on both axis and if so do you secure your camera cables to the center of your scope balance point because if the just hang at the end the will cause drag and that will not help at all   

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Don' be misled by focal ratio. A 10" F5 will have a focal length of 1270mm, a 80mm F7 is 560mm focal length. Anything over 1000mm focal length can be a challenge whereas well under is a much better chance of success.   :icon_biggrin:  

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On 8/21/2017 at 13:52, toxic said:

is your scope balanced on both axis and if so do you secure your camera cables to the center of your scope balance point because if the just hang at the end the will cause drag and that will not help at all   

sorry i missed your comment, yes I've balanced the scope in both axis but as for the cables, this was something i was worried about. I was worried about me securing them too tight because they might get tangled or pulled. Do you know of any good particular way of securing them? thank you

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Sounds to me like the PA isn't quite right..you say it finds the targets well enough..star alignment isn't the same as polar alignment as with PA you're aligning to the earths axis..as pointed out the pds is going to be more of a challenge because of its size and weight..a 80ed less so as it has a shorter fl..smaller so it's less effected by the wind and weighs far less..

I'd run the drift alignment tool and get the PA nailed.. if you still have drift problems then you can then look elsewhere...

What does the RA and Dec errors say on phd?

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The trailing is likely to be the focal length, if the polar alignment is a bit off then the longer the focal length the greater the trailing. As such a shorter ED would help with this. Additioanlly the balance on the 250PDS could be a problem.

Is the refractor the f/7.5 option ?? If so then a scope alone will make a difference on exposure length but they are generally used with a reducer/flattener and so this would lessen the difference.

I know someone with a 250 who images, they are looking at a more substantial mount then the EQ6 they presently have, reason is they cannot get the results and the mount appears to be the problem.

If you are talking the ED form of the scope (doublet) then reconsider for a triplet, either a skywatcher or another such as William Optics - maybe the GT81 if 80mm is the desired size. There are cases of CA on the doublet and you do not want to find out that you are in this situation. The smaller still WO Star 71 seems to be getting good results at present - that will be one huge difference in size. I would additionally expect a triplet to be around the f/6 value so closer to the 250PDS.

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I agree with ronin regarding focal length and PA, your PA @ 1200mm FL really must be spot on.

I don't see any problem with a decent doublet though, the evostar ED80  is a great, popular scope with no CA worth mentioning and i recently bought the equinox ED80 which is also very well colour corrected with no CA and it's a 500mm F/6 so pretty quick, it does require the optional FF but then so might a triplet.

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