Saturday, June 5, 2010

6" F/5 Dobsonian: Finished (at least my part)

We cut up the 1/2" (actually a little less Baltic Birch) on a friend's table saw (thanks!), and finally glued most of it up yesterday.

There is a hexagonal box around the center of gravity of the telescope tube.  That was hard to do.  I thought I had the table saw set to very close to 30 degrees from vertical (with a precisely cut piece of cardboard) and I thought we were getting 0.01 inch precision on the length of each cut, but the box was a bit too big, and the angles didn't quite close up.  To compensate for the size being too big we cut two sides a bit smaller, and we shimmed between the box and the tube where needed.  Here is the hexagonal box drying, held together with a giant rubber band (red) and Duck tape (silver).

The rocker box, on the other hand, came out really nicely.  The round parts were done with my fixed router.  We did need to use a home-made drum sander (plug cut by hole-saw on a bolt spun by a drill, with sandpaper hotglued--unfortunately, eventually the sand paper came off) to expand the sides.  I also glued 1/16" bondable PTFE pads to the sides under the rockers, and used the rockers themselves to press the pads down.  The clearance is pretty small--we'll see if any problems develop as the wood ages.

The rocker base is very simple: just a round piece of plywood, with three round legs (2.5" plugs from using a hole saw to cut the lightening holes in the rockers), and PTFE pads near the legs (for stability).  Bricks used for clamping both the pads and the legs.

Finally, today, with the kids' help I added the rockers to the tube.  I also added little wooden thingies, with PTFE lining inside the rocker holes on the box to keep the rockers from falling off.

And here is the finished scope.  Well, finished, except for finishing the wood, which is for my friend.  Interestingly, the tube and the primary cell are really stable.  Even though we did all that stuff with the tube, and the primary cell was removed for much of it, when I put the primary cell back in, although secondary collimation was off (I think that single-stalk secondary holder may need an upgrade eventually), primary collimation was almost exactly spot-on.  I also put some silicone on the outside of the inner edge of the focuser tube, where the Crayford focuser's bearings go, so the tube can't be racked too far out.  To prevent it from being racked too far in, the set-screw for eyepieces is located so it'll hit a wooden piece eventually.  To remove the focuser, I guess one either has to twist the focuser tube past the rockers (not good for the aluminum of the tube) or remove the secondary and get it out from the inside with the set-screw removed.  Now my friend has to finish the wood.

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