Tom
I'm basing my reply on an 8in F7 truss/upper cage study .
On purpose , I used the weakest stuff I had , to better
evaluate flexing , and in particular , on what axis .
3/4 in sq. sawn pine truss , 1/4 in luan plywood rings .
For the rings , the stiffness is due to the ring width ,
rather than to its thickness .
My rings were ID = 9.0 , OD = 10.25 .
Again , think where the ring stiffnes is needed ,
and of course , degrees of freedom .
The top of the truss attachment , CAN't move up/down ,
as they are attached firmly to the mirror box . So we
needn't worry a lot about the ring flexing up/down ,
as attaching to the truss stiffens the ring for us .
It is the axial flexing of the ring we need to reduce,
as the truss CAN move IN/OUT .
I suggest a study using thin but wide rings ( 1.25 inch ? ) for you , and REPLACE the upper cage struts , with axial 1/4 plywood "spokes" .
I was planning a double ring assembly , just high enough for the focuser board .
Oh , and the PINE truss tubes ?
I learned much , about flexing from that little experiment .
The truss material serves only to create fixed triangles ,
the material strength is of secondary concern .
It is FAR more important , to tie the 3 points of each
truss pair ( triangle ) rigidly . Wood screws through
pine , failed miserably , glue was perfect , though not
at all transportable .
I then placed the scope frame horizontally , and clamped the 1ft mirror box to my cast iron table saw table , with
the remaining 4ft frame extended into air .
I hung a 10lb weight on the focuser end , and measured total flex , compared to unweighted posistion .
All I can say , is the frame didn't budge , but the entire
scope moved , due to the soft pine mirror box sides , squirming beneath its table clamps .
Truth be told , the ring assembly ,only needs to be stiffer that the force requires to move your scope on its bearings .
Keep us posted on your thoughts .
Wayne
