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Serious, overnight doubts about spars and mirror torque around the
beams has suggested I examine the cardboard tubes, plywood rings and well
separated, alloy beams design. The present, single spar does not lend
itself readily to Dobsonian mounting. The center of gravity is in mid
air above the beam making altitude bearing attachment a difficult design
exercise. Whatever I choose to add to the beam would add weight which
has to be lifted and carried out to the mounting. I can no longer underestimate the importance of reduced OTA weight after suffering an agonizing back injury just from lifting
my telescopes! Separating the two or four beams adds enormously to the rigidity of the OTA. Altitude bearings are readily attached with the ability [via clamping] to slide up and down for precise OTA balance and alignment. The cardboard tubing is thermally neutral compared with alloy pots. The beams are not close to the light path in this design and have low thermal mass and excellent exposure to the air.
Though bulkier to mount, the OTA should be much easier to carry than the single spar type. Which always wanted to invert itself so that the cells were pointing straight downwards! Carrying the spar OTA was a major chore and very risky when negotiating doors. It was not only very long but had seriously overhanging pots at each end.
Beams surrounding tubes and rings will have a better defined overall dimension and be balanced in all planes. The downside is adding the weight of two more beams. Though weight savings will occur with removal of all the heavy, channel sections.
The
image shows a mock-up to determine OTA stiffness using only two side
rails. Rather than waste birch plywood I used the drilled rings from the
earlier [failed] dowel OTA experiment. I simply cut diametrically
opposed slots for the beams in the widths of the four rings. The interesting point about this arrangement is the opposed leverage against the two rings ensures rigidity. The rings cannot twist on the beams provided the cells are firmly constructed [glued] as one unit.Using just two side rails for a Dobsonian mounting would probably suffice. The altitude bearings could easily be clamped to the side rails to allow longitudinal balancing of the OTA. Even relying only on friction between the beams and slots and rings/tubes the structure already seems very stiff compared with the ease of twisting the pots around the original duplex spar.
If I proceeded with this design and cut new 12mm (1/2") birch plywood rings I would prefer not to drill holes in the alloy beams. Given my habit of constant updates and improvements some means of clamping would probably be best. The alloy beams are a commercial product and readily available but rather too expensive to spoil them cosmetically by drilling and screwing before the design is finalized. Opposed wedges could be inserted behind the beams with a length of studding pulling the wedges together to jam the beam against the cardboard tubes. The plywood rings would be glued to the cardboard tubes by then to avoid the whole thing falling apart during transport.
I am still trying to decide whether a central 'cell' similar to the other two, to support the altitude bearings would be beneficial [for stiffness] or merely add more weight. The balance point will almost certainly have changed relative to the single spar/pots design.
Attempts to distort the simple arrangement of two beams and two cells showed that it was immensely stiff in the vertical plane but could be slightly "lozenged" laterally. Whether this would alter the optical path once lateral pressure has been removed [for pointing at an object] is an unknown. The leverage applied for pointing, or nudging the tube for tracking, would be roughly halved by the midway position of the altitude bearings. Slightly increased stiffness is likely to be gained by the need for lateral location from the rocker box.
Adding further beams, at 90 degrees to the other two, would provide an incredibly stiff OTA in all planes, but it would obviously be heavier. In both weight and appearance. There is something rather special about an interesting and/or slender design. Though all four beams could be fitted with altitude bearings to allow the OTA to be rotated to bring the eyepiece to the most comfortable position.
I'm not sure I would try to mount the two beam OTA equatorially because of the degree of overhang. Certain orientations might also throw diagonal loads onto the parallel beam geometry. No stiffening can be applied which would intrude into the light path. Otherwise I could just add some diagonal wires for triangulation. The four beam design would obviously be far superior in this respect for equatorial mounting. Having an OTA which could be instantly mounted either equatorially or as an altazimuth Dobsonian would/might be an attractive combination.
Click on any image for an enlargement.
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