How can I build a Dobsonian mount
Posted 10 June 2009 - 09:02 AM
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.
Posted 10 June 2009 - 12:20 PM
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'?
Posted 10 June 2009 - 04:22 PM
Posted 10 June 2009 - 09:07 PM
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.