I'm converting an old Dall-Kirkham f/20 scope from a massive heavy tube weighing 12Kg (bare) to an open truss design. The only bits I'm recycling are the primary and the secondary/spider assembly. The design I'm looking to implement is the one shown in this page from Mr. Royce's home page.
http://www.rfroyce.com/DK%20OTAs%20struts.htm
He uses a 40x3mm square box section as a central rib with trusses to the front and back made from what looks like a 25x25mm channel profile (2 or 3 mm thick). The point of attachment of the channel at every location requires a section of the sides of the channel top be cut away allowing a flat surface to be screwed into the rib/end-ring etc. The alternative to the use of channel is 20-25mm tube but the connection machanisms are necessarily more complex. The weight of a metre of 25mm channel is probably much the same as 20mm tube, I'm guessing.
But the question is whether the cutaway section of channel doesn't represent a major point of weakness in the structure.
Apart from the basic frame (10Kg), the remaining weight will be mirror/mirror cell 10Kg, Secondary/spider 3Kg, Baffle tibe/focuser assembly with connecting flange say 3Kg - all up about 25-26Kg. The point of attachment to a mount would be a Losmandy Bar screwed to the underside of the rear section so those relatively few bit of aluminium would have to carry the whole weight.
Does anyone have any thoughts about the choice between channel and tube in this sort of a structure?
Peter.
