Optimal Design and Method of Cassegrain Instrument Unit for 1.2 m Sun Yat-sen University Telescope

SYSU (Sun Yat-sen University) 1.2 Meter Telescope is a R-C Cassegrain optical Alt-Az telescope, the Cassegrain instrument unit is mounted on the telescope's mechanical derotator, which is required to feed more than five terminal instruments by a central switching mirror with a switching accuracy less than 1 arcsec. It is a great challenge to design, accommodate and align so many instruments in such limited focus space and realize so high switching accuracy between different instruments under different observation modes at the same time. This paper explores design process problem and find the way in detail for the Cassegrain instrument unit.