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How can I build a Dobsonian mount

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#1 Jimmy2K63

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Posted 10 June 2009 - 09:02 AM

Just the kicking the idea around this morning about possibly building a Dobsonian mount for my Cave 10" f/5. I'll have it here shortly and I just know that before I get the observatory built, it's going to be a bear to pack around the mount so I thought seriously about just building a Dobsonian to use for a while. This way I can attend the star party this fall on a budget. I think one sheet of 3/4 inch plywood would be enough to get me a pretty decent start on it all. I am very good with woodworking so that part doesn't really concern me much.

I am thinking about building a simple and removable non-destructive tube cradle that can straddle the balance point. I think this would be simple enough to lay out and cut out/assemble. Building the base should be pretty simple too - but having never seen the bearings on one of these - how would I build the bearings for the cradle/base interface as well as the altitude axis in a relatively simple way? The azimuth shaft is what concerns me as it has to be tight enough to rotate without any kind of wobble,. Anyone ever used any old phonograph records or used CD's as bearings on one of these things?

If you have photos, feel free to share.

#2 John Carruthers

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Posted 10 June 2009 - 12:20 PM

The centre bolt doesn't have to be very precise unless you intend to fit encoders, in fact if the bottom bearing were arranged as an inverted cone it would self centre and not need a bolt, picture a ball scope sitting on 3 points.
Where you don't want any slack is between the tube and the side bearings, most annoying if the tube creeps back a fraction when you let go. I originally had 2 teflon wedges here but now use 2 small ball catches, they allow the tube to lift in/out but are stiff enough to take out the play.
A rough rule of thumb is to make the bearing size similar to the mirror diameter but it's by no means crucial. Lots of scope (?) for interpretation and creativity.
Have a read of Kriege & Berry's book, 'The Dobsonian Telescope, A Practical Manual for Building Large Aperture Telescopes'?

http://www.willbell.com/TM/dobtel.htm

#3 coopman

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Posted 10 June 2009 - 04:19 PM

Google is your friend:
http://pages.sbcglobal.net/raycash/
See the links at the bottom of the page.


#4 coopman

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Posted 10 June 2009 - 04:22 PM

The mass produced 8"/10" Dobs use a 22" dia. ground board & bottom board. The height of the side boards will be more or less determined by the length of the tube, its center of gravity + some distance to make sure that the bottom of the tube clears the bottom board. You can buy molded plastic side bearings for the OTA from Hands on Optics - they would screw right into the tube.

#5 Jimmy2K63

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Posted 10 June 2009 - 09:07 PM

I did a little sketch today to get some ideas and made a trip to the hardware store and lumberyard to see what they had. I've figured out a way to make a cradle that won't mar the tube in any way and is pretty simple to remove for those times when I want to install it on the GEM in the future observatory. Thing is the CG won't change from either mount so once located it would be useful for both mounts.

I found some furniture moving pads about 2 inches in diameter that I thought would work on the base plate, 3 of them at 120 degrees, lock it all to adequite tension with a bolt and lockwasher assembly. I briefly considered spring loading the bolt and that may be an option. 3 friction points guarantees that all will be contacting at once.

The cradle will be a two piece unit radiused to the tube itself and held together with 8 small 90 degree brackets bolted, the cradle itself lined with felt, much like the present straps on the GEM. For the altitude bearings, I think a pair of V notched sides lined with some teflon would work - the wheels then only contact at 2 points. I am hoping that the tube assembly is heavy enough to provide enough friction, and if not I will make some mods with some type of a side-thrust assembly.

I do have some advantage - our club built a Dob for our old 10 inch when we got our new scope, but it was ugly. I've also used and studied the XT10's out pretty closely and I like their setup. I figure with plywood and all I can do it all for around $50 - going to use 3/4 inch.

Give me a couple of months to get it all together, and I'll share it in pictures. If nothing, I will learn by trying :-)

I may take a ribbing from the guys over in the Classic's forum, but hopefully not too bad after they see that I have not destroyed or changed the original tube and mount in any way.






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