10" f/8 Altazimuth Dobsonian OTA ideas.

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I am currently re-pondering my options. Perhaps using a Porsa primary cell framework with 30mm diameter truss tubes. The problem then is knowing how "tall" to make the primary cell to allow the direct attachment of altitude bearings without having the truss poles and cell framework to hand.

Substituting two more, 2 meter long, builder's straight edges would leap that hurdle at one bound. Leaving the whole truss problem and altitude bearings irrelevant. But, requiring yet more serious thought as to lightening the secondary cage without loss of stiffness. Perhaps I should utilise a single ring with a right angled surface for finder and focuser support? Or mount the focuser to look through one of the alloy beams? 

Well, I finally made a decision and bought two more beams. They sell for just under £20 equivalent. Which is good value compared with the price of 2m of Porsa square tubing or buying 2m lengths of round tube online. These builder's straight edges are also readily available in most large DIY outlets.

I weighed the pair to remind myself that they are dead on 3 lbs each. [~1.4kg] So 4 beams will be 12 lbs bare of cells, mirror, focuser and finders. Since I will attach the beams to the sides of the cells I shall have complete freedom to mount arcs or full circles for Dobsonian altitude bearings.

Hanging the beams temporarily in place, using cord on the horizontal OTA, suggested I choose a new focuser angle to match an altazimuth mounting. Should I rotate the existing cage to make the focuser horizontal or [perhaps] 45 degrees above the OTA axis for greater viewing comfort? It really ought to be on the left [looking up the tube from the primary end, to make finder use more sensible.

I bought some suitable M6 stainless steel coach screws, some nuts and matching washers while I was in the builder's merchant. I still have several lengths of 30cm [12"] diameter, cardboard tubing left to mock up a new secondary cage. So might explore that route instead of drilling new holes in the existing alloy cage. It won't matter if I make a complete mess of attaching the beams to the cardboard tubing while I try out some different ideas. Even placing the fixing screws will mean the hidden, central web must be taken into consideration when drilling the beams. It would help to resist crushing the beams. 

I'd also like the altitude bearings to be able to slide and clamp anywhere long the beams for achieving OTA balance. Just like I did with the equatorial mounting. This saves having to use adjustable sliding counterweights on rods. Or attaching separate counterweights to the OTA. I know from long experience that anything heavy which has to be carried around, or attached later, is just another impediment to regular telescope use.

iStar optical are/were offering skeleton tube refractors. Though they use 3 beams and multiple CNC'd metal support rings which also act as the light baffles.

Using four builder's straight edges would provide a nice, stiff and lightweight OTA for a long focus [classical] refractor. The beams come in lengths up to 3m straight off the shelf. Which would perfectly suit an 8" F:15 refractor made with a four beam skeleton tube some 10' long! How well the Fullerscopes MkIV mounting would cope I have no idea. Though there's always the Beaconhill MkII mounting for such huge telescopes. I would probably need a much taller pier to avoid grovelling on my knees when pointing at the zenith. Well, one can but dream! ;ø))

Today I weighed the secondary cage [metal pot] and found it to be 6lbs without an eyepiece or finder. So then I immediately cut an 8" length of the 12"/30cm cardboard tubing to replace the pot. I tried to make some tightly fitting 1/4"/6mm thick plywood, external stiffening rings. This proved that cutting out the bores of the rings with an electric jigsaw was not nearly accurate enough!

So then I spent some time making a new circle cutting jig for my Bosch router. This will have a capacity of about 24" diameter circles and be infinitely adjustable. A centre pin is provided by Bosch for making circles using the adjustable fence but I found it all but useless. To make smooth, accurate circles requires a really sturdy centre pin in a pre-drilled centre hole. Certainly not just a point resting in a shallow dimple in the wood to be cut. Making the circle cutting jig larger than my immediate needs will allow me to cut large radius, altitude bearing arcs later on. Once finished, the router jig made some smoothly round circles which fitted the cardboard tube exactly.

The image shows one routed ring in the foreground against one sawn ring in the background. The cut-outs in the scrap plywood rings are for the alloy beams. I am still experimenting on a replacement secondary cage so don't want to waste Baltic birch ply at this early stage.

Try to ignore the deliberate mistake in routing my old workbench top. Last time it was with a circular saw so I'm not really improving with time.

Routing a ring is not quite as easy as it looks. You can easily have a perfect bore or a perfect circumference. Getting both requires some means of retaining an immovable centre hole for the circle cutting jig. Nor can you use clamps as the router base will hit them. Ideally it needs a large, router cutting board. Made with a sacrificial top surface which will still accept screws to hold the centre circle cut-out perfectly still aftwer the router cut is completed. For the occasional cutting, of just a couple of rings, this seems too much like hard work.

A temporary lash up with the beams fitted in the ring notches suggests there is real potential to this new design. The shoulders of the notches are critical in resisting twisting of the rings around the beams. Adding a similar central section, like the secondary cage, would help to stiffen up the OTA without affecting its moment. The use of cardboard tube and plywood has the advantage of thermal neutrality in changing temperatures. Though the alloy beams do remain.

The iStar TCR [refractor] is certainly attractive and remarkably lightweight but doesn't readily lend itself to an altazimuth fork mounting.  A Dobsonian needs lateral supporting beams for fixing the altitude bearings on either side. The [light coloured] mounting clamps can be seen attached to one of the longitudinal beams or stringers. Notice how all the reinforcing rings are bored to match the light cone and thinned to save weight. In light polluted observing conditions a tubular sleeve of black cloth could be added. Much like the shrouds applied to truss Dobsonian OTAs.

Each beam would need to be bolted to the rings and/or tubing. I envision a rectangle packing piece between the beam and the cardboard tubing to avoid local crushing. Perhaps with two parallel lengths of dowel attached to spread the crushing loads into the cardboard tube. The rather weak plywood, which I have used so far, would be much improved by substituting Baltic Birch, multi-lamination plywood. Perhaps in 10mm, or even 12mm thickness to keep weight down at the secondary cage end. The rings would need to be glued firmly to the cardboard tube to ensure there is no rotation between these components or all stiffness would be lost. I am unsure how much "meat" should be left at the bottoms of the notches to resist the ring crushing the cardboard tube or vice versa. These suggestions are to further strengthen the beam's ability to avoid twisting against the tube/rings and to avoid crushing forces when the beams are bolted to the rings. It is not as if the forces are trying to ovalise the tube. Rather, that the crushing force is applied to the rings via the bases of the notches.

Click on any image for an enlargement.

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