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Question about SkyWatcher collapsible Dob.

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#1 maugholddave

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Posted 18 July 2019 - 02:49 PM

Hi all

I'm new to these forums and telescope ownership, and have a question about the scope I have bought. It's a SkyWatcher 12" GOTO Dobsonian which I am still familiarising myself with, and am curious as to why there are 2 positions that the tube can be opened up to? One is approximately 2 inches short of full extension. Does it have any effect on the focal length, and which is the correct position? I've only used it on 3 or 4 occasions so far, and have always had it fully extended. I'm waiting for another opportunity to try the scope with the tube extended to the shorter position, but just wondered if anyone else would know?



#2 vdog

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Posted 18 July 2019 - 02:55 PM

The shorter setting is for use with a binoviewer to bring it to focus without a barlow or corrector.  I toyed with the idea of getting a binoviewer, but that idea went by the wayside and I never use that setting.

 

Welcome to CN, BTW.


Edited by vdog, 18 July 2019 - 02:56 PM.

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#3 maugholddave

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Posted 18 July 2019 - 03:08 PM

Aaaah! That makes sense. Why don't they put that info in the manual I wonder? Thanks vdog.


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#4 kfiscus

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Posted 18 July 2019 - 03:16 PM

Welcome!  That's a nice scope that you've chosen.



#5 mic1970

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Posted 18 July 2019 - 03:23 PM

I love mine, but.... On my ten inch, I have to collimate the mirrors every time I raise and lower it.  However, my buddy has the 14", and he does not.  So, I'd check for a bit until you know. 



#6 maugholddave

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Posted 18 July 2019 - 03:29 PM

Thanks Ken. I've wanted a scope for about 30 years and finally had enough saved to buy a decent one. It is probably a bit advanced for a first scope, but I hope to keep it for a good few years and make the most of the dark skies we get here. Really happy with what I have seen with it so far, although I suffer with the dreaded astigmatism which is going to force me to spend a bit more on eyepieces with better eye relief (or save up for Televues with their dioptrix adaptor)



#7 maugholddave

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Posted 18 July 2019 - 03:39 PM

Collimation hasn't been too troublesome. Because of where it is stored and where I set it up, it does seem to need collimating every other time I set it up. I bought a cheap laser collimator (which itself had to be collimated, before putting it in the scope) and this does make it a fairly quick operation.



#8 mic1970

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Posted 18 July 2019 - 03:51 PM

If I may, from one newbie to another.... Use the laser, but get a simple cat's eye eyepiece to really nail it down when you are out for a full session.  Very simple to use. 

Collimation hasn't been too troublesome. Because of where it is stored and where I set it up, it does seem to need collimating every other time I set it up. I bought a cheap laser collimator (which itself had to be collimated, before putting it in the scope) and this does make it a fairly quick operation.


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#9 RaulTheRat

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Posted 18 July 2019 - 03:54 PM

The other reason you might use the lower position is to get the focal plane far enough out above the tube for a camera, but since you'd only be using a camera on a dob for short exposure stuff like planets and wouldn't have all the added length in the light path of an OAG, filter wheel etc etc then I guess that even the fully extended position might work fine with a camera.
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#10 maugholddave

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Posted 18 July 2019 - 03:55 PM

Thanks mic1970, I'll look into that. Is there anywhere that I can find out more about it, as I have never heard of this method?



#11 maugholddave

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Posted 18 July 2019 - 04:01 PM

Thanks RaulTheRat. Not planning on any photography as yet. I just want to get comfortable with the scope and do some general observing for now.



#12 mic1970

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Posted 18 July 2019 - 04:24 PM

See Newbie.  I found out they are called a collimation cap.  I bought one from a friend for $10 new, but can only find what I have on a Euro website.  https://www.firstlig...tion-cap.html. 

 

I know you can make them from an old 35mm film canister.  Check the ATM forum. 

 

Here is a nice metal one on Amazon. I will say that may be worth it.  The plastic one is really light and easy to drop and lose.  I'm down one already.  https://www.amazon.c...ateway&sr=8-9. 

 

Just search collimation cap or Cheshire on CN's search engine.  There is a ton.  I use the laser to dial in and the cap to finalize.  I'm told the Cheshire is more accurate then the laser unless you really dial in your laser's collimination and a solid eyepiece holder connection.  So to this newbie, it is the best of both worlds.  

 

Video: https://www.youtube....h?v=BaDgFl3joOI

 

 

Thanks mic1970, I'll look into that. Is there anywhere that I can find out more about it, as I have never heard of this method?


Edited by mic1970, 18 July 2019 - 04:27 PM.


#13 vdog

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Posted 18 July 2019 - 04:36 PM

I love mine, but.... On my ten inch, I have to collimate the mirrors every time I raise and lower it.

Yup.  Worth checking every time even if you leave the truss up between sessions.




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