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Old 18-02-2016, 05:18 AM
Ru1s (Chris)
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Skywatcher Collapsible Dob Synscan + Synguider ?

Hi Everybody,
I am about to purchase a Skywatcher 14" collapsible dob goto scope.
looking at the synguider ccd autoguider to accompany this scope.

I enjoy a little astrophotography with my current 8" unguided no goto but would like to extend my interest in this hobby to my new scope,

Just wondering if anyone has had success with this kind of setup or if there is a better route on this scope i should follow.

Thanks all.

Chris
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Old 18-02-2016, 06:59 AM
dimithri86 (Dimithri)
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Hi Chris,

The image still rotates when tracking with a dob, so your exposure is limited. Using a guider might make it easier to take multiple exposures and stack.
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Old 18-02-2016, 07:30 AM
glend (Glen)
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You can take quick shots of very bright objects like Jupiter, the moon, etc but for long exposures where guiding is required like for galaxies and nebulas you will need an eq mount with goto and guide capabi,lity.
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Old 18-02-2016, 11:37 AM
raymo
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Guiding with a Go To dob would be an exercise in futility. You will be limited to very short exposures [due to the aforementioned field rotation]
that won't go very deep, and won't need guiding. You can easily manually take some exposures and stack them in Deep Sky Stacker. I reckon you'd be better off buying a premium eyepiece or two with the money. Were you aware of the limitations of field rotation before deciding to buy the Go To dob?
raymo
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Old 18-02-2016, 11:44 AM
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rustigsmed (Russell)
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hi chris,

i'd recommend getting a HEQ5 equatorial mount. you could in theory put your tube from the dob ontop. or you could get an another imaging newt and it would still be cheaper than getting the 14 goto dob.

i started off doing dob shots some are here https://www.flickr.com/photos/803366...57634243605994 they were good at the time for a dob but can't compare to an HEQ5 even unguided. mine were taken with the 12" the 14" has a longer FL which will make capture a bit more difficult.

Russ
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Old 18-02-2016, 02:49 PM
Ru1s (Chris)
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first off wow, you guys are quick.
dimithri86 -> yep generally the idea.
glend -> thanks
raymo -> no was unaware of field rotation. but must be better than my 8" OTA on a homemade dob mount unguided right ?
rustigsmed -> perhaps the 8" may find a new home on a HEQ5 sometime down the track. and OMG those photo's are amazing. but the increased light gain should equal shorter exposures required right ? ( over the aforementioned 8" ) or my neighbours 6" mak-cass celestron nextar 6se. being a alt-az same issue with field rotation right ?

and again wow thanks all for speedy info.

ok so i know my 8" my exposures have to be really short like 2 seconds MAX. ( no auto guiding )
i also know my neighbours 6se we can get 10 - 15 sec exposures before we get star trails be it either from drive errors or field rotation i cannot say.

in conclusion I am coming out of this thinking that perhaps the autoguider whilst not ideal now.
1 > Would come in handy for adapting the 8" on a EQ mount later down the track.
2 > The added guide capabilities would keep the 14" steadier and could up exposure to 15 - 30 sec's perhaps ?

If the 2nd is true and 30 sec exposures is achievable then deep sky stacker and photoshop should manage the rest.

is this correct or is my thinking out of alignment ?
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Old 18-02-2016, 04:24 PM
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rustigsmed (Russell)
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I'm pretty sure you can't connect guide errors as there is no input into the dobs... increased aperture doesn't equal shorter exposures it means more resolution. the focal ratio really dictates exposure time.

they track pretty well when setup properly depending on how far away from the zenith and pole it is. i could get 30 seconds but found it more consistent to do 20 second exposures. any longer and the field rotates (unless you were at the north or south celestial pole).
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Old 18-02-2016, 05:13 PM
raymo
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You seem to be getting the wrong end of several sticks Chris.
As Russ said,the focal length of a scope dictates the length of exposure that you can use without getting star trails.
I assume that your 8" is f/5, 1000mm focal length. That allows about
half a second near the celestial equator, and around 2 to 3 secs near the celestial poles. Pretty useless, I think you'll agree.
The 14" has a focal length of 1650mm, so simple maths show that the useable exposure time will be substantially less than your 8" allows,
[with the motors turned off, of course]. When using the Go To you will
still of course be limited to well under 30 secs by the aforementioned field rotation.
As Russ pointed out, I'm pretty sure the Go To dobs don't have an
autoguider input socket, as they are not designed for astrophotography.
The 6SE is a Schmidt Cassegrain not a Maksutov. It is f/10 giving a
focal length of 1500mm, so it also would allow much shorter exposures than your 8"[when it's drive is turned off, of course.]
raymo
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Old 18-02-2016, 05:40 PM
dimithri86 (Dimithri)
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Just keep doing the astrophotography the way you have been do it, and save your money for some eye pieces. Two good eyepieces can revolutionise your scope, and will probably give you alot more enjoyment, than what a autoguider on your setup will give.
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Old 18-02-2016, 06:48 PM
Ru1s (Chris)
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rustigsmed -> 20 seconds is sufficient for my needs right now. and awesome news. even better that no autoguider input means less expense right now.

raymo -> your correct the 8" is f5 and yeah upto about 1 sec exposure on orion nebula before unacceptable trails. I have managed reasonable stacks but was after longer. ( 15 - 30 seconds would be good ). the 14" is a f4.5 so shorter focal length = longer exposures ? my 8" isn't motorized at all. And yes schmidt cass and yeah f10 and 10 - 15 secs exposures tops.

dimithri86 -> thanks for the encouragement.

so here's what i am taking away from this conversation.

1 > The 14" goto being better ( the fact it's goto / tracking ) as opposed to my 8" full manual scope ( and not the most stable being a home made mount ), or my neighbours (not mine ) 6se ( so only have access sometimes and they want to use it too , it _IS_ theirs after all *giggle* ) is leaps and bounds better than my current setup,

2 > First and foremost _Visual_ use is primary the pictures are just nice to have. so it ticks that box. it _can_ do photography much the same as the 6se but due to the shorter F/ratio collects light quicker/better resolution ? (the 14" that is) so better than my current pictures.

3 > I'll have much better access to the 14" as it will be mine.

4 > I can (later when time and money permit) get a EQ mount for the 8" for proper photography.....

am i on the right track now ?
and thanks all for your patience and understanding.
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Old 18-02-2016, 07:16 PM
raymo
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You still have it wrong Chris.
Your 8" f/5 [200mm] dob has a focal length of 1000mm.
1000mm focal length divided by 200mm mirror diameter = 5 hence f/5.

14"[355mm] has a focal length of 1600mm.
1600 divided by 355= 4.5 hence f/4.5, so the 14" has a 60% longer
focal length than your 8", and so will allow only much shorter exposures than the 8".
You are right about the 6SE; at f/10 it is very "slow" photographically.
The 14 would gobble the light up compared to the 6SE, and as you say
with much better resolution.
raymo
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Old 18-02-2016, 08:24 PM
Ru1s (Chris)
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raymo -> right, I was thinking the F number was the all important number not the focal length. thanks for the clarification on that.

and thanks all for your understanding and patience.

i can't wait for first light, still about a month away , then i have to prey to the weather gods for clear skies, i should be lucky enough to catch orion before he dips below the western horizon before sunset.
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