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DIY Dobsonian Telescope Mount

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Dr_GMJN31/01/2021 11:43:02
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996 forum posts

All, I thought I'd share this telescope mount I've been building. Every year for about the past 5 years I’ve been saying “I’ll get this done for February and get some use out of it in the clear winter nights” At last it’s done, now for the clear nights...
I bought a Meade 10” Starfinder second hand back in 1997, but never really used it because of the hopeless equatorial mount which vibrated at the slightest touch, and had huge stiction issues. It didn’t even track properly. I therefore decided to take the plunge and design and make a Dobsonian Mount.

I started by reading a few astronomy websites and gathering some info, then got a spec. together and fired up the CAD:





One of the biggest problems was getting or making an inexpensive large diameter tube clamp (that stops the OTA falling through the cage, and allows balance point adjustment). Between starting the design and actually making it, I’d got an FDM 3D printer, so it was a very easy task to design and print one, incorporating some standard parts off Amazon. I had to make it in four identical quadrants due to printer bed size:





A colleague has a home-made CNC router, so I exported the CAD to 2D, and he cut all the parts from MDF:



I designed it to be self-jigging, and it all slotted together with a bit of sanding and some wood glue and screws. I then sprayed it with three coats of exterior satin varnish.

I lined the cage with some old snooker table baize to make the action of adjusting the tube nice and smooth. I’ve since added a ring of baize to the face of the cage for the printed tube ring to seat on:



My colleague also routed the other problem items - the elevation trunnions - from hardwood blocks. They just needed a final flick on the ML7 to ensure they were true, then I glued textured Formica edging around them to reduce stiction. I also printed some retainers to go over the ends of the edging to prevent it ever springing off. The Formica runs on machined Teflon blocks mounted in printed retainers. The aluminium discs stop lateral movement, and rest against felt pads glued to the inside of the bearing block retainers:



continued...

Dr_GMJN31/01/2021 11:43:15
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996 forum posts

I previously requested ideas for the azimuth bearing over on the "General" forum, which I've incorporated into the design. I used a 390mm diameter ball-raced lazy Susan slew bearing. I got it from Simply Bearings who do a range of them. I assumed it would cost a fortune, but it was about £38. The white outline was a bit of a mistake during routing the base. It's filled and flatted, and has no effect on function.



Then I carefully washed the mirrors which were a bit grubby (following instructions on YouTube). Rinsed with distilled water, they look a lot better:





I'd previously painted the inside of the tube matt black, and also fitted a much better quality focusser to replace the rather sketchy plastic original:



I got a laser collimator, which was good fun to use, especially when I put a snuffed candle in the tube to generate some smoke. The kids loved turning the adjusters and watching the beam move about until it was bang on:







The Telrad reflector sight still worked on its existing battery after at least 15 years in storage. I did treat it to a new one though:



I made the lower base from two 12mm ply sheets glued and screwed together. I would have used 1” ply, but couldn’t source any locally, and didn’t want to travel without good reason, so I made do with what I could carry within walking distance of home. Then put three adjustable feet on it from Amazon, and secured it to the lower mount with an M10 stud, thrust bearing and nylon nut. the latter was tightened just enough to stop separation of the lower mount structure from the circular base.

I also printed a quick eyepiece rack, and that’s it:









Ultimate goal is astrophotography, so the current w.i.p. Is a motorised equatorial wedge table. Should get about an hour of tracking out of it.

I've not used it yet - too cloudy! I really hope it works ok in 'real world' conditions.

Thanks for looking!

Roderick Jenkins31/01/2021 12:26:14
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2064 forum posts
555 photos

Very nice yes

May your skies be clear.

Rod

Dr_GMJN31/01/2021 14:18:37
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996 forum posts
Posted by Roderick Jenkins on 31/01/2021 12:26:14:

Very nice yes

May your skies be clear.

Rod

Thanks Rod!

old mart31/01/2021 17:41:30
2829 forum posts
178 photos

Clever design, your printer must be pretty large. Sealing the MDF is very important to retain its dimensions. What decides the pivot point, is it the COG of the tube assy?

Edited By old mart on 31/01/2021 17:44:47

Dr_GMJN31/01/2021 19:33:31
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996 forum posts
Posted by old mart on 31/01/2021 17:41:30:

Clever design, your printer must be pretty large. Sealing the MDF is very important to retain its dimensions. What decides the pivot point, is it the COG of the tube assy?

Edited By old mart on 31/01/2021 17:44:47


Thanks Mart.

You slide the tube in and out of the cage until it’s balanced on the horizontal pivot axis (depends on what equipment is in and around the eyepiece), then lock the tube ring. The tube ring stops the tube falling through. The vertical rotation axis is directly under the centre of the horizontal pivot axis.

My printer is the usual size for a home machine - bed is about 250mm square IIRC.

The MDF has 3 coats of varnish, so should be well sealed for what it is. It’s stored in the house, and will be outside only for the duration of an observing session.

Cheers!

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