2" shaft mounting Pt 19: Yet another rethink.

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While I have been waiting for the worms and wormwheels to arrive I have been drawing and thinking hard. Without knowing the thickness of the wormwheel on its boss I could make no fast decisions on where to place them nor how much room they needed. 

I was not very happy with the very deep Polar Axis framework in the previous design. [Pt.13]  The bearing support arms were so long that they provided a lot of leverage. With the likelihood of increased flexibility unless built massively. So I decided to reduce the depth of the 'U' shape considerably. Which didn't leave any room for a counterweight for the offset Declination assembly to pass through the PA frame.

The answer, of course, lay in having a weight extension arm clamped above the PA support frame. The RA wormwheel could also sit above the frame. Allowing an even shallower frame. The downside to this layout was nothing would be able to pass inside the PA frame. The limit, before a meridian flip was required, would be when the Dec housing hit the PA frame.  

I was tempted to saw up some slotted angle iron to build a trial PA frame. Then thought better of it and decided I'd use glued up 2x4's for the prototype. Or I could bolt together some 2x4 channel to make 4x4 'I' section.

The ideal arrangement for all sky access and compactness is a conventional German mounting. The Polar Axis 'skeleton' is shown alongside propped up at approximately the correct 55 degree angle. The distance between the bearing flanges is about 50cm.

Doubts were cast by one person on the CN discussion thread  on the stiffness of the flange which would join the Declination assembly to the Polar Axis. This criticism had seriously undermined my confidence in the German mounting due to all the weight being cantilevered off the top of the PA shaft. Subsequent purchase of the Tollok locking bushes has helped to rekindle my faith in the German mounting's potential. The German mounting is by far the most popular design for most classical refractors.

I have some 180mm, 7" diameter, alloy bar. A thick 7" diameter disk could provide a large surface area if the Tollok bush was expanded within it while simultaneously locking itself onto the PA shaft. The flange would rest on top of the RA wormwheel. With a further flat surface on top the PA housing. Though the wormwheel is not essential and could be placed elsewhere. The top disk would provide a larger surface on which to mount the Dec assembly. Teflon/PTFE would be useful between the surfaces of the disk to reduce friction. The Tollok bush has to be placed there anyway. So any worries about increased overhang are moot. The Bush can be sunk within the top disk to reduce any added overhang.

The images show a full sized mock-up of the conventional German mounting arrangement. The tube rings shown are for a 20cm [8"] diameter, main tube. The saddle is 70cm, 32" long. As is the Declination axis. The PA is 60cm or 2' long with 50cm, 18" between the polar bearings. The battens are just for temporary support for the picture. The PA Tollok bush [just visible between the Dec studs] would be buried in a thick 7" disk for reinforcement. By good fortune I obtained the massive lump of aluminium bar for small change at a flea market. Slices just fit in my 4-jaw chuck. 

After struggling with countless SketchUp drawings I have given up on trying to make a compact cross-axis design. They are too limiting of  the ability to see much of the southern sky without a meridian flip. No matter what I tried the declination assembly struck the Polar frame before the tube had managed much more than 90 degrees of RA movement.

A full cross-axis mounting needs towering support pillars and a huge polar axis for a large refractor. I did some mock-ups of those with lengths of timber and it would need to be absolutely colossal to support an 8' long refractor!  The telescope tube still has to be mounted as high as a German mounting to ensure comfortable overhead viewing. The main advantage I can see with the full cross-axis is isolation of the supports from a raised observing platform. For which it would be almost ideal. Long lengths of 10cm, 4" square pressure treated timber are available at modest cost. This would be far easier to build than a towering concrete pier which would need isolating from the platform.