Design, manufacture, measurement, and installation of the gross mirror cell of the LAMOST Schmidt plate

The gross mirror cell of the LAMOST reflecting Schmidt plate supporting 24 hexagonal sub-mirrors with an alt-azimuth mounting has already been in routine operation for two years. During observation, the gross mirror cell must hold good structural stiffness and stability, meanwhile, lightweight, good transportability and accessibility for maintenance is implicitly required. This paper describes structural design principle, manufacturing, transportation, installation and measurement of the gross mirror cell. Based on diverse investigation of design scheme and aided with finite element analysis, we achieved an optimum mirror cell, which is actually a hybrid of space frame and truss structure of about 5.4 m by 6.3 m, which meets very well technical requirements with lightweight, high stiffness and clear accessibility. The truss part is supported on the space frame part with a few carefully chosen nodes. Bolted sphere connection is selected for truss nodes. That only commercial steel material is used and medium precision is defined lead to low manufacturing cost. During assembly at workshop and restoration at site, 3D laser tracker has been used to not only measure the 3×24 working nodes in the top layer truss but also aid the installation process of the mirror cell. The final measurement result is 0.3 mm RMS within the required 0.5 mm. The mirror cell was manufactured in Nanjing and transported to, ~1300 km away, LAMOST Xinglong station northeast to Beijing. In this paper, we also share special consideration for the long distance land truck transportation and experiences got during large-sized precise part like the gross mirror cell installation to 25 m high pier in situ.