ATM 4 tubes truss structure questions...

["Benoit Schillings" <benoit@be.com>]

Well, I'm trying again to build something which will NOT be a 6 or 8
tubes truss !

I saw the pictures at the Fairborn observatory

See 

http://schwab.tsuniv.edu/t4.html


I really want to build the OTA of my 15 inch with that kind of design,
a 4 tubes structure making a pyramid like stucture holding the 
secondary
mirror of my cassegrain.

The secondary assembly weight about 1.6 pounds, I will use some 0.5
inches carbon/carbon tubes for the assembly and some spherical washers
at the top and bottom of the tubes.


I would like to receive any advice I can on that design... I understand
that this is not as stable (structurally) as a normal 6/8 tube truss, 
but
I find enough advantages (including the fun of expermenting) in that 
design.



I notice on the picture that they use additional bracing between the 4 
tubes,
can someone give me the rational behind that ?
Will I have some rotational stability problems without that extra 
bracing ?


FYI, the distance between primary and secondary is 106 cm (about 40 
inches).

The base is 18 inches and the top plate joining the tubes is 4.6 
inches.


Thank you for any help.


                -- Benoit