2" shaft mounting: Pt7: PA- mounting base considerations.

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The entire and very heavy mounting must be supported on the pier in a manner which is as rigid as possible. Yet still allow the polar axis to be tilted to match the latitude. I have had good success with a heavy set-up using a humble turnbuckle. These are readily available at very low cost for tensioning wire fences. Better quality turnbuckles are available in polished stainless steel at several times the price of the galvanized variety.

The turnbuckle applies tension in very fine increments by rotating the body. The combined left and right hand threads turn against matching L&R threaded hooks or eyes. Naturally it will need the polar axis housing to be hinged or pivoted somehow. The studs and shafts of my [skeleton] bearing housings allow at least a 20mm bar, pipe, screw or stud to pass easily between them. So no silly ideas, like the MkIV Fullerscopes mounting, thank you. Which required that two small and short pivot screws must be tightened in putty soft, aluminium castings to prevent flexure of the polar axis casting in the thin supporting, cast fork.

A nice, big, fat, threaded crossbar can be tightened with a nice big spanner.[US.wrench] This has two advantages. First it ensures security and longevity of the crossbar threads. More importantly it also allows the cheeks [tines] of  the fork to be solidly locked to the polar axis housing by friction alone applied over a wide area. Once the nuts are tightened the polar axis housing becomes an integral part of its supporting fork. The fork is in turn supported by the polar axis as one whole unit. No silly and spindly little altitude tabs supporting the whole 'kit and caboodle for' us!



The image of the SWEQ2 has been overlaid with potential flexure modes. These are only shown in elevation and there will also be 'cross' flexure modes. All of these must be eliminated in a serious, heavy duty mounting. It takes only one mistake to introduce flexure at one point in a mounting. That will be enough to make the image wobble in a breeze or when focusing at high powers. Even a dovetail fitting can be a disaster waiting to happen. It fits in a very short casting and relies on a side screw to hold it fast.

The local pressures on the relatively soft castings must be enormous. A long saddle firmly attached to the declination shaft via a solid, steel flange and numerous bolts should avoid this commonplace flexure bottleneck.

Imagine a small Dobsonian rocker box. Usually the sides are separated by the telescope. So no clamping between the supporting side boards is possible. Turning the telescope in azimuth causes the side boards to splay and distort slightly. Our solid polar axis housing, with its massive threaded cross bar, can lock up the whole arrangement preventing any twisting or flexure at all.

It may be highly convenient to have a dovetail but it has no place carrying a large refractor. You simply cannot ignore the enormous leverage of a long telescope applying massive pressure so close to the fulcrum. The width of the head which accepts the dovetail is the distance from the fulcrum to the applied force.  A long telescope applies effort at up to 48" from the fulcrum. A typical dovetail housing might be only four inches across. 1lb of force applied 48" from the fulcrum = 48 lb/inches. The pressure on the far side of the dovetail is 48 x 4 = 192 lb/inches. That is only a 1lb push at the eyepiece. You could lever a large rock out of the ground with a 4' crowbar! Or even lift a car!  

Our load spreading measures mean that the mounting's base and fork can be built from humble [birch] plywood without a single qualm or restless night. Provided that the crossbar applies pressure through roofing washers then the plywood won't and can't be crushed. It is backed up inside the fork blades by the multiple, birch ply laminations of the polar axis housing. The entire mounting base and polar axis become a rigid monocoque. One unable to flex or twist by mutual reinforcement.

Dob style, altitude bearings will provide the necessary load spreading to avoid damage to the laminated plywood tines of the PA's supporting fork. We certainly aren't amateur enough to [ab]use the crossbar itself as the tilt pivot alone in a bare plywood hole. That might tend to make the heavy mounting head south over time whenever the crossbar nut [or nuts] were loosened to adjust the polar attitude. A suitably large, galvanized coach screw would do just as well as any fancy, threaded crossbar. Though domed brass or stainless steel nuts might look well if you can find them in seriously large sizes.

The supporting mounting fork blades will be firmly attached to a thick, plywood base flange. Once the three components [base/fork/PA housing] are locked together by the compression bar there will be no bottlenecks left to flex.

Click on any image for an enlargement.

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