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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, lightweighted 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.
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