Optical compensation of the Chinese Large Solar Telescope instrumental polarization

Abstract

One of the main tasks for the Chinese Large Solar Telescope (CLST) is to measure the solar polarization with high accuracy and sensitivity. However, as a classic Gregorian configuration telescope with an alt-azimuth mount, the telescope system itself will introduce instrumental polarization. Even though there will be a calibration unit on the second focus, it is still hard to calibrate the instrumental polarization all the time because it will change constantly with the rotation of the telescope. In this paper, we proposed a project to compensate the CLST instrumental polarization with an optical compensator. This compensator, which consists of seven flat mirrors, can in theory compensate the CLST instrumental polarization for any spectral lines. When the polarimeter is not working, it can be removed from the system. We simulated the project using software called the Advanced System Analysis Program (ASAP). The result showed that this unit can compensate the instrumental polarization completely without considering the post-focal instruments.