The reflecting Schmidt plate MA of LAMOST is with 5.7m x 4.4m reflecting area and consists of 24 segmented hexagonal sub-mirrors. Each sub-mirror is 25mm in thickness and 1.1m in diagonal. To correct the spherical aberration of the primary mirror, during observation, the aspherical shape of MA should be changed in every 1.5 minutes. To achieve the good image during observation, the active support system of MA will not only create the correct off-axis aspherical shape on each sub-mirror, but also maintain the co-focus for all 24 sub-mirrors.

This paper presents the studying design with finite element analysis and experiments on the active support system of MA, including its axial and lateral supports, force actuators, optimization of the stiffness of the force actuator, sub-mirror cell, the mirror support structure etc. There are 30 force actuators and three position actuators, which support each sub-mirror and connected by sub-mirror cell. Total 24 sub-mirror cells located on the top of the MA main support structure. All force actuators work as both active and passive supports for each sub-mirror. It showed that the support system is complex but should work properly within the optical requirement.