Mount

Box Frame

The first step was to build the box frame to hold the two tubes.  Jim Mettler took charge of this part.  The frame was made from 3/4" square aluminum tube (1/8" wall thickness) held together with machined fittings press-fit into the ends.  The fittings were made very slightly oversized, with the corners of the square pegs rounded to avoid splitting the square tubes.
 
(Left) Jim Mettler uses his press to attach a fitting to a tube section.
(Right) A close-up of a corner fitting about to be pressed into a tube section.
After a few hours of cutting aluminum tube lengths and pressing the fittings into them, the basic frame took shape.  Then we added two 1/8" aluminum sheet plates to hold the altitude bearing, and since one section of the frame has to be left out so that the frame can be moved in altitude when on the pier, more 1/8'' aluminum sheet was used as a gusset in the back to keep the two sides from sagging towards each other.


The frame (with the altitude bearing added in the middle).


Pier


The pier is from a 16" Meade Starfinder Newtonian.  The pier is on semi-permanent loan from the Prairie Grass Observatory.  (Cat included for scale :)


Bearings

The following parts make up the bearings:  At left is a section of 4" PVC pipe that slides into the pier.  Two snug wooden spacer rings made by George Gourko provide a tight fit (only one is shown-the other is already in the pier).  At the top of the pipe is a threaded adapter.  In the center of the picture is a 4" PVC "T", sliced in half, that forms the heart of the mount (Jim Sattler's idea).  The third ("stem of the T") opening is threaded, and screws on top of the pipe to form the azimuth bearing.  Heavy grease is applied for smooth movement.  At right is a 4" diameter aluminum tube that will be cradled in the top of the "T" to form the altitude bearing.  This tube will be mounted in the box frame between the optical tubes, so it will be sort of an "inside out" version of a standard Dob mount.

For the altitude bearing, wooden disks made by George Gourko were pressed into the ends of the 4" diameter aluminum tube.  These were drilled off-center (left) to allow a vertical adjustment for balancing the scope.  But the off-center holes means there will be some torque on the tube, so sandpaper disks were glued to the disks (right) for better friction when the bearing is mounted between the side plates in the box frame.  To allow for horizontal adjustments, slots rather than holes were cut in the side plates for mounting the bearing.
 
Teflon pads were added to the T, and a stop was added to make sure the mount wasn't accidentally "unscrewed".


The nearly completed pier and bearings.

Finally, to finish the project, the inter-ocular adjustment system was added.

The handle in the middle rotates a part (below) that moves two struts attached to the upper mounting rings of the optical tubes, at a point roughly opposite their pivot points, so that the tubes swing apart.


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