Materials and structural design for lightweight primary mirrors

A deflection analysis for seven potential primary mirror materials is presented for loading due to gravitational or external forces. Based on this analysis a finite element model of a plano-plano mirror is developed for three mirror materials including aluminum, Zerodur and an Mg particulate-reinforced composite. Comparison of mirror deflection according to the finite element model to interferometer-measured deflections of a Zerodur mirror indicate good accuracy is obtained from the model. Using the calibrated finite element model various geometrical light weighting options are evaluated. These include the fabrication of between three and six circular pockets on the back face of the mirror. It can be concluded that while both pocket depth and diameter contribute to mass reduction of the mirror, mass reduction through increases in pocket depth are more advantageous than increases in pocket diameter, as this results in greater mirror stiffness for the same mass reduction. Also, an increase in the number of light weighting pockets for the same mass reduction has less impact on the mirror optical performance.