Design of a Polarization-Analyzer Control System for the Lijiang 2.4m Telescope

Polarimetry is one of the four major types of astronomical measurements (spectroscopy, photometry, polarimetry, and imaging). A polarization analyzer is an effective tool in polarimetry. We have realized a closed-loop polarization-analyzer control system, which consists of a step motor and an absolute encoder. The step motor is controlled through the TCP/IP protocol, and the encoder is controlled through the RS485 serial communication. The system has a polarization-angle measurement error of 0.42°, a stepping angle of 22.5°, and a slew range of 0~360°. These sufficiently meet the requirements for polarimetry. A traditional polarization-analyzer control system is of open loop and contains a step motor and a Hall device. Through the Hall-effect switch the initial position of the polarizer can be determined, and then commands are sent to the step motor for designated angles, such as 45°, 90°, 135°, and 180°. It is easy to be operated to realize its basic function but it has large errors because of lost steps of the motor, the number of which is hardly known. In case when the system is restarted after unexpected power out, the current position of the motor cannot be specified and the entire system (including the step motor) has to be initiated again. We use an absolute encoder to replace the Hall device in our closed-loop polarization-analyzer control system. We can inquire the absolute encoder to retrieve the information about the current position of the polarizer. The systematic error can be controlled by the encoder also, and the positioning accuracy depends on the resolution power of the encoder. There is no need to worry about any power off for the system because the position is displayed in real time. Overall, our polarization-analyzer control system has very good positioning accuracy and robust performance. It is convenient for polarimetric applications.