Topology optimization of a double-sided space mirror based on additive manufacturing of SiC

The integration of primary mirror (PM) and quaternary mirror (QM) has potential applications in on-axial four-mirror anastigmat space telescopes for low mass and compact structure. Aiming at fabricating silicon carbide (SiC) space mirrors by additive manufacturing, this paper presents the topology optimization method of a monolithic double-sided mirror with controlled surface errors. For optimization, the RMS deformation of PM under axial gravity is set as objective, and total weight is adopted as constraint. The evolution of surface figure errors (SFE) in topology optimization is explored for insights into optical performance. The relationships between SFE of the two optical surfaces are discussed, and the composite performances of mirror are evaluated with regard to polishing pressure load. A verification model is established by finite element method to evaluate the design results. Results show that by imposing an extrusion constraint and proper support boundary conditions, double-sided mirror with self-weight deformation superior to that of conventional isogrid mirror can be achieved. The surface errors of both PM and QM converge with similar trends in topology optimization, and the surface precision of QM is always better than that of PM. Thus, the precision of PM remains the major design concerns. Besides, the topology optimization only optimizes the aberrations with relatively high intensity. On the other hand, the mirror exhibits obvious print-through effect due to nonuniform rib distribution generated by topology optimization. A secondary optimization by adding cathedral ribs can alleviate this drawback with minor modifications to original design. Finally, a prototype is printed and machined to testify the feasibility of double-sided SiC mirror.