Preliminary Research of the Light-Weighted Secondary Mirror with Passive Supports for the MUltiplexed Survey Telescope

The MUltiplexed Survey Telescope (MUST) is a 6.5-meter aperture wide field spectroscopic survey telescope built by Tsinghua University. In the optical system of MUST, the second mirror is designed as a hyperbolic flat- convex reflector with an effective aperture of 2.4 meters. In order to achieve high imaging quality under different zenith angles, light-weighted mirror structure is adopted to effectively reduce the weight of the secondary mirror and depress the deformation of the surface figure. In this paper, a preliminary finite element model of the second mirror with multi-point passive support is established and numerical simulation is utilized to investigate the effects of different light-weighted schemes. In the simulation, parameters of the mirror structure are taken into consideration, including the distribution and the structural forms of the light-weighted holes, the back plate and the rib, and the distribution of the passive supports. According to the optimization results, a light-weighted secondary mirror with 36-point axial support is finally achieved to significantly reduce the weight by more than 70% and effectively improve the surface accuracy to 20nm RMS value. To our best knowledge, this is one of the largest aperture light-weighted secondary mirror with passive supports and the presented light-weighted design could provide a reference for the future development of 2-meter class secondary mirror.