Re: ATM Serrurier Truss Flexure

[Andy Saulietis <iss@pvtnetworks.net>]

Don C writes,

>A Serrurier  truss is not super rigid and has flexure just like all trusses.
>Mark Serrurier relized that flexure could not be eliminated totally. So he
>designed the truss so for any orientation of the scope the secondary and
>primary flex by the same amount so that collimation is maintained.

Not well understood by ATM's is that the actual Serrurrier truss
requires a truss structure at both ends of a central box that holds the
dec or elev axis..one truss supports the secondary and its cage, the
other supports the primary mirror. If properly designed, the optical axes of
both the primary and secondary mirrors stay co-aligned, ie both mirrors deflect
by the same amount without decollimation. Not a trivial design exercise 
unless the
trusses are the same length and have the same load at the outer ends. The
typical single truss Dob must thus be rigid enough such that the decollimation
stays withi the desired tolerances.


Andy Saulietis
ISS Enterprises
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