TMT Tertiary Mirror System (M3S) is different from other telescopes. There are many instruments on Nasmyth platforms both sides, some are not located on elevation axis. When the instrument that does not locate on elevation axis is working, M3S should rotate in angles to reflect light ray to the instrument. The rotating angle varies following the telescope position. M3S should be capable of pointing and tracking, rotating with the observing position, which results in complicated load cases. In this article, Rotator assembly is designed according to M3S design requirement, and key techniques involved in Rotator are studied. At last, performance of Rotator and M3S are analyzed. The mains points are as following: (1) The azimuth axes schemes of present large aperture telescopes are summed up after looking up plenty of documents. (2) Researches are developed on supporting scheme, driving scheme and position measurement on the basis of M3S characters. The mathematical models of different types of bearings are built, and the performance under different zenith angles is obtained. Dual-motor driving scheme are adopted to satisfy the requirements of structure. Dual-motor driving principle is studied, and the driving structure is designed, the control results are simulated. Gear transmission error is evaluated by Monte-Carlo method. Because of the large diameter, a tape encoder is adopted, the accuracy and usage of the encoder are explored. A test were carried out to check the effect of four readheads and Fourier harmonic analysis to improving angle measurement accuracy. (3) Key components of Rotator are optimized, the mass distribution is more reasonable. As the interface between M3S and tower would affect the M3S performance, kinematic interface and non-kinematic interface are put forward. After evaluation, the non-kinematic interface affects less to the bearing mounting surface than non-kinematic, so the former is confirmed. The axes accuracy are discussed. (4) The static and dynamic performance of Rotator are analyzed by Finite Element Analysis (FEA). As the performance of Rotator when it is assembled to M3S is more important, the FEA is also done to M3S when the system is under different zenith angles. The deformation and stress response are acquired. As TMT observatory is a seismic zone, the ability of M3S to resist certain intensity earthquakes are analyzed. The protection means are discussed too. At last, the structure response to wind is simulated. In this article, some key techniques involved in azimuth axes are studied on the basic of TMT M3S design. Performance of the system under different zenith angles are evaluated sufficiently, that would be very helpful guidance to M3S design. Some conclusions can be applied to general large telescope design.
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