Optically, the first order effect of this rotation is zero since the secondary mirror is nominally on the rotation axis. Small pointing shifts could result from asymmetry, and path length changes appear as a second order effect. The stiffness of the structure depends on the strut dimensions. Since shadowing is not affected by the strut depth this may be quite large. The most critical choice is the width which strongly affects the blockage and stiffness.
Conventional structures use two, three, or four struts.
The choice of the number of struts may also be influenced by the symmetry of the backing structure. For minimum blockage with a given strut width the fixture points should be at the rim of the primary mirror to avoid blockage of the spherical wave. However, since the strut length is reduced, the strut width may be decreased for the same stiffness and the blockage penalty is minimized. Narrower base attachment makes the structure much more rigid as the bending stresses are decreased in favor of longitudinal compression stress. Structure used in the calculations.