Appendix A: Derivation of the Secondary Sag as a Function of Telescope Altitude . One-half of the secondary support structure consists of the two rods shown in Figure 9 plus the opposing rods on the other side of the secondary cage. The total tension force on a single rod can be decomposed into a thermal component and a base tension that is set at the time when the rods are originally tensioned with the turnbuckles. ... The axial force balance equation is then given by . ...