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Yes, ive often wondered what effect the AL mounting rails had. My RCOS tube has the rails mounted on al clamps , one at the primary mirror end and the other about half way up the tube, I wonder if the clamp allows minor expansion due to temp?. The secondary mirror is mounted directly to the tube further up. I dont have focus shift problems at all.
The RCOS truss OTAs have rails hard mounted along the short tube (al?) part before the final CF truss connected to the secondary, I dont think they have focus drift either.
The CDK looks to be all CF, the short tube part and both truss sections, but again, the tube part is hard connected to an al rail.
As you say, it makes you wonder why they bother with CF tubes at all, unless the clamps are designed to allow "slippage"
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