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Just a further correction....
The Serrurier tube design is not just a "truss" design.
The detailed write up on the design from the 1930's shows the true nature (and elegance!) of the design.
The top and bottom trusses as well as the joints should allow the top and bottom "rings" to deflect BY THE SAME AMOUNT and also maintain the top ring parallel to the bottom ring. This means that any deflection only results in a change of focus position without rotation.
These criteria are obtained by varying the cross section of the trusses and their relative lengths...
( I was at Grubb Parsons works during the late 1960's when they were actually testing a "Serrurier Truss" telescope tube.)
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