Structural optimization of a space mirror to selectively constrain optical aberrations

Structural optimization of a 1.2 m diameter space mirror is carried out using OptiStruct tool of HyperWorks 12 to minimize mass and constrain optical aberrations. Zerodur is used as the material for the mirror. A typical open-back mirror of the same diameter and material weighs around 90 kg. For optimization, constraints were imposed on defocus, coma and trefoil aberrations. Aberrations like piston and tilt are caused by rigid body displacements of the mirror can be compensated by on-board control mechanisms and hence ignored for this study. Also, astigmatism is controlled by the design of Mirror Fixation Devices (MFD). Since MFD design is out of the scope of this study, astigmatism is also ignored. Optimization runs are carried out for various boundary conditions. The optimized design weighs 63 kg and has acceptable optical performance. Suggestions are also presented to further minimize the mass while maintaining an acceptable level of optical performance.