Image-Field Rotation and Control of Counter Rotation for the Spectrograph of the 1m Solar Telescope of the Yunnan Observatory

The 1m solar telescope of the Yunan Observatory has a spectrograph as one of its main terminal instruments. The telescope has an altazimuth mounting structure and a Gregorian optical system. In the process of tracking the sun, the solar image on the slit plane of the spectrograph rotates around the main optical axis because of the kinetics of the mounting structure and mirrors. Such image-field rotation should be compensated for the spectrograph to work properly. We first give an in-depth study of the image-field rotation on the slit plane, and analyze the variations of rotation angle and acceleration of image-field rotation with the solar hour angle. We subsequently provide key parameters about position detection and driving motors of the counter-rotation system based on the specifications of image-field rotation and requirement on precision of counter rotation. We finally give a design of the servo control of counter rotation for the spectrograph.