Author Topic: Swapping out SW focuser  (Read 1478 times)

Andy

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Swapping out SW focuser
« on: September 06, 2016, 12:23:22 PM »
Hi guys. I recently swapped out the standard SW focuser on the 12" truss dob for a Duel speed version. Problem is the 2" adapter I had for original does not fit into the DS. U popped a2" ep straight into tube sans adapter but couldn't resolve any stars. Is there a specific bit of kit I need to buy to get up and running again? I know a few of you have/had SW dobs before and I'm hoping someone has fixed a similar issue themselves.
As always any advice and suggestions are gratefully received
Cheers
Andy
300mm Skywatcher Truss Dobsonian
80mm Celestron wide field refractor
Telrad and 9X50 RACI finder
Meade 5000 32mm/Q70 38mm/Skwatcher Nirvana 82 degree 28mm, 16mm, 7mm/Explore Scientific 100 degree 20mm/ Vixen LV 9mm /Televue Delos 4.5mm/ Televue Radian12mm and x2 Astro Revelation Barlow
Nikon D5000

iam4208

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Re: Swapping out SW focuser
« Reply #1 on: September 06, 2016, 02:31:16 PM »
I've never done it, but compatibility is a common problem.  If you follow this link, you'll see a compatibility table for Crayford focussers, the issues that can occur and solutions.
If it's not a Crayford you've got the advice may not be exactly what you need, but it could set you on the right road.  Sorry, I can't be of any more help.

http://agenaastro.com/gso-crayford-focusers-with-telescopes-compatibility.html
Sky-Watcher Skyliner-250PX FlexTube Parabolic Truss-Tube Dobsonian with Baader Hyperion 36, 21, 13 and 8mm EPs and a 2x Barlow.
Telrad and RACI finder scope for finding objects the good ol' fashioned way:-)

Andy

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Re: Swapping out SW focuser
« Reply #2 on: September 06, 2016, 04:26:53 PM »
Thanks mate. I'll have a wee look at that later tonight when I get home. Nothings ever easy in astronomy eh
300mm Skywatcher Truss Dobsonian
80mm Celestron wide field refractor
Telrad and 9X50 RACI finder
Meade 5000 32mm/Q70 38mm/Skwatcher Nirvana 82 degree 28mm, 16mm, 7mm/Explore Scientific 100 degree 20mm/ Vixen LV 9mm /Televue Delos 4.5mm/ Televue Radian12mm and x2 Astro Revelation Barlow
Nikon D5000

Lorne

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Re: Swapping out SW focuser
« Reply #3 on: September 06, 2016, 07:45:49 PM »
You may need an 2" extension tube, 35mm & 50mm are the most common sizes. Next time you are set up, focus all the way out then try pulling the EP slowly out the focuser whilst looking through it, you will probably find you are able to reach focus doing this.

watson

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Re: Swapping out SW focuser
« Reply #4 on: July 29, 2017, 02:14:21 PM »
I just read a post about measuring the back focus distance of a Newtonian.
So I measured the back focus distance of my Explore Scientific ED 127mm APO. A refractor.


I had to add a lot of extension pieces to get the eyepiece to focus


With the 40mm and 28mm (2") eyepieces the back focus came out at 175mm.
With the 16mm (1.25") eyepiece the back focus came out at 167mm.



I used the two 37mm long (threaded M48) ES extensions and two 40mm long (slide in clamped) ebay extensions and some of the rack and pinion to get to focus.


When I replaced the eyepiece with the camera (Canon 450D with 44mm inset from EOS face to focal plane) I had to use the two 37mm screw in extensions and one 40mm ebay extension and a lot of R&P extension to get the camera into focus. The T2 to eos is  9mm thick. Total added extension is 2*37mm + 40mm + 9mm = 123mm


I have since bought a 2" to M42 52.5mm extension that screws into the eos to T2 adaptor.  That makes all the connections screw ins except for one 2" slide in clamp.


With the diagonal in place and visual viewing with eyepieces, I only need one 37mm ES extension to get to focus.
« Last Edit: July 29, 2017, 02:25:16 PM by watson »
Sky-Watcher eq6 pro
Explore Scientific 127mm f7.5 Apo
Orion UK europa 250mm f4.8 Newtonian
Canon 450D, not modified (yet)
2" 40mm & 28mm and 1.25" 16mm Explore EPs
Altair 2" diagonal and 2" Barlow