7" f/12 iStar refractor 30: The 180mm goes altaz?

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I have just realised that the Orion[UK] Dobsonian mountings use their own "rolled" rings to support the altitude bearings. I have two of these rings to nicely fit the 8" diameter of my 7" refractor OTA. These rings are remarkably light [1lb each] compared with the plywood-packed, traditional "Skywatcher" type rings @ 2lbs each. Which means that I can have the trunnions permanently fitted to the OTA without adding much weight to carry out to the waiting altazimuth fork. Since an altazimuth mounting does not tip sideways it needs no closed rings to avoid the OTA falling off.

Now I need to design some altitude bearings to fit onto the Orion rings which weigh as little as possible. A minimum diameter of 8" seems reasonable for the trunnions. Increasing the diameter applies just enough extra friction to avoid tube balancing problems. The trunnion bearing pads can always be moved apart to increase friction if necessary. My 5" refractor used 6" PVC rings against PTFE/Teflon pads but only because that was the size of tube I could most easily obtain. Some builders add "Formica" to the edge of plywood arcs to form the bearings. I have yet to come across any form of laminate in DIY outlets in Denmark. Perhaps I just haven't been looking hard enough?

There is a well-boring company not too far away. They always have a large skip full of potential PVC pipe off-cuts with which to make altitude bearings. I must try to avoid the usual massive disks of plywood which are traditionally used to support the bearing surfaces. In 3/4", or even thicker, birch plywood can add considerable weight even in 8" diameter. Yet the support for the stubs of tube, which form the actual bearing surfaces, want to be stiff enough not to distort or flex. Particularly when the telescope is moved over a small angle to bring an object back into the field of view.

It is here that the [usually] buttery smooth, low friction movement works so well on a Dobsonian telescope mounting. One wants just the right amount of dynamic friction compared with static friction. So that the telescope does not drift freely nor have so much friction that it causes judders as the bearing "un-sticks" in small steps. There are favourite laminate materials which have been proven to have the perfect bearing materials in combination for this very purpose. Some builders swear by waxed bearings to achieve the perfect level of friction. In comparison with ball or roller journal bearings the Dobsonian bearing can provide the perfect telescope movement. Even allowing very high powers to be used to follow an object effortlessly.

My earliest 5" refractor mounting, built decades ago, moved with a pull of only 1lb in all directions up, down and sideways until it reached the zenith. Only here did friction rise above the normal. It felt absolutely magical in use despite the simplicity and low cost compared with building a mounting with bearings. In fact it took years before John Dobson's ideas were universally accepted. They could not bring themselves to take the giant leap of faith away from their expensive, precision mountings and costly [commercial] OTAs.

Building a suitable offset fork is not just a matter of adding thick lumps of plywood to a suitably thick plywood base. The base joints want to be reinforced with webs or box sections to avoid splaying [or swaying] of the fork sides when the telescope is being moved. There is noting worse than having mechanical backlash when only a tiny angle change in pointing angle is wanted. The frustration of not being able to perfectly center an object with the slightest nudge or pressure is best left to the equatorial. Though one could apply slow motions to a Dobsonian they really should not be necessary in a good mounting design.

Nor should the azimuth bearing in the base be made too small. An undersized base will tend to tip too easily simply through having too small a footprint for unconditional stability. The smaller the footprint the larger the angle forced on the base through the slightest sinking into soft ground. If that tipping coincides with telescope movements then the valuable stability of the Dobsonian design is lost.

For the same reason one should use four feet instead of the theoretically perfect three. The base will always flex, or sink, just enough, to provide vastly greater stability than having only 3 feet. Only a rigid 3-legged stool on a hard, uneven floor will enjoy greater stability over a normal 4-legged one. Piers ought to have four legs rather than three for the same reason.

The radius to the tipping line, drawn between any two feet of  3- legged support [of exactly the same length of side as a 4-legged object] is always much smaller. See the image above where all sides are the same length. It takes little imagination to see how a top heavy object can fall outside its center of gravity with a 3-legged pier. While any lopsidedness remains stable with a 4-legged device. There is almost always enough flexure to ensure all 4 feet feel the same pressure to resist toppling on anything but a perfectly hard surface. Climbing onto a 3-legged stool to reach a high shelf is not recommended! A quadricycle is far more stable than a tricycle during cornering. Which is why there are so many cars with the normal quota of four wheels and so very few three wheelers.

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