29.04.2021 The hideous reality of building a cross-axis mounting!

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Thursday 29th 52F max and overcast.

I brought a 3.2m length of 50mm PVC pipe up to the observatory to represent the cross-axis, polar axis.  It was quite a struggle to get it inside the dome!

The northern bearing support would be fully 3m [10'] above the observatory floor. The south bearing would be at floor level. Not far from the south wall. This would allow the Dec axis to be at the same height as the present DIY GEM. 

Finding suitable donor materials for such a long polar axis would not be easy. My scrap, aluminium tubing stack has two lengths of 1.60cm. Twice that makes a polar axis. 

The downside is that the tubing is rectangular 100x200mm. [4"x8"]  So I can't use the 50mm bore flange bearings from the big GEM. The square flanges are 145mm on a side. 

Not that this is a serious problem. I could use smaller shafts and smaller flange bearings. There are no, heavily cantilevered loads, of the GEM, in the stubby shafts of a cross axis. One could even use a cylindrical, plain, northern bearing resting in pads for light, applied friction.

50mm shafts and bearings can still be used in the declination axis. Though it would require a sturdy construction to join the lengths of 4x8 solidly together. While allowing the declination shaft to be mounted at right angles.   

A better [?] alternative might be a tubular, square cross-section,  plywood, polar axis. Tapered to taste, from centre to ends, in truly classical, observatory style. Easily made to any desirable dimension.  Mitred longitudinal joints for cosmetic acceptability. No exposed laminations on the edges to give the game away. Low thermal mass while subjected to hours of sunlight. So no thermal convection currents rising across the field of view.

The declination axis junction is also easily arranged in plywood. The weight and solidity of plywood would enjoy the natural damping of most, wood-based materials. Tapered, tubular sections of the PA would enjoy immense stiffness. Particularly when subjected to the modest loads of my few, amateur sized telescopes. 10" f/8 on the opposite side of the PA balancing the 7" f/12 and the 6" f/10 solar H-a telescopes.

Do I have a plan? 😎

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