Fig. 1. Composition of the derotator

Fig. 2. Finite element model of the anti image rotation mechanism

Fig. 3. Deformation of circular grating in horizontal position

Fig. 4. Measurement of the derotator accuracy

Fig. 5. Derotator positioning accuracy

Fig. 6. Integration of the derotator and telescope

Fig. 7. Derotator is turned off（a） and derotator is turned on（b）

Fig. 8. Star spot labeled 1（a） and star spot labeled 2（b）