Practically all solar phenomena are more or less relative to the solar magnetic field. It produces relatively stable structures like sunspots or prominences and is responsible for spectacular dynamic phenomena like flares or coronal mass ejections. However, the generation, amplification and destruction of magnetic fields remain poorly understood. The knowledge of its magnitude and direction is crucial for interpreting measurements of other parameters, and it can be measured usually by a polarimetry at some special spectral lines, which should be sensitive to the Zeeman effect. To answer what physical mechanisms are responsible for heating the corona, what causes variations of radiative output in the Sun, and what mechanisms trigger flares and coronal mass ejections and so on, many large aperture solar telescope have been developed (such as VTT, GREGOR, NST) or are beening developed (such as DKIST, EST), and the Stokes polarimetry is their most important observational device for determining the magnetic field. The Chinese large solar telescope (CLST) with a 1.8-m aperture is a classic Gregorian configuration telescope with an alt-azimuth mount. It would be the second largest solar telescope in the world for a long time. And it is the main task for the CLST to measure the solar polarization with a high accuracy and sensitivity. However, as a classic Gregorian configuration telescope with an alt-azimuth mount, the telescope system itself will introduce instrumental polarization. And it will also change constantly with the rotating of the telescope. Therefore a calibration unit which produces light of known polarization states is necessary to measure the Muller matrix of the system and apply the correction numerically on the measured Stokes vector.  
    Professor Changhui Rao’s group, in Institute of Optics and Electronics (IOE), Chinese Academy of Science (CAS), is making progress in the polarization calibration technology of solar telescope, and published a paper in Opto-Electronic Engineering. In this paper, researchers introduced the polarization calibration method and proposed a calibration progress. Since the telescope is rotational, symmetric down to M4, the position of the secondary focus F2 is appropriate for a calibration unit because the components before it do not contribute to the Muller matrix. F2 is an extremely useful position in CLST. It should be used not only for polarimetric purposes but also for internal alignment. Thus the calibration unit becomes a more versatile device. Then they designed the structure of CLST calibration unit with two turnplates. The calibration unit optics equipped on these turnplates will consist of a rotatable linear polarizer, and two rotatable achromatic quarter wave plates which are in use alternatively for visual range or infrared observations. And there are also some other space on the turnplates to realize the internal alignment purposes.

Fig. 1  Configuration diagram of CLST calibration unit

Fig. 2  Structure diagram of CLST calibration rotator

About Team
Professor Changhui Rao’s group, in Institute of Optics and Electronics, Chinese Academy of Science, has rich experience in study of solar observation with high resolution. They have solved several key techniques in this domain including thermal control, tomography imaging and MCAO, etc. Currently they are busing to build the 1.8 m Chinese Large Solar Telescope (CLST), which is one of the largest ground-based solar telescopes in the world. Their harvests in solar observation have been published in Astrophysics Journal, Astronomical Journal, JATIS, Optics Express, Solar Physics, and so on. They also do some interest works in polarization imaging, and the latest research results have also been published in Optics Letters, IEEE Photonics Journal, Optik, and so on.

Yao Benxi, Rao Changhui, Gu Naiting. Polarization calibration unit design of 1.8 m Chinese large solar telescope[J]. Opto-Electronic Engineering, 2018, 45(11): 180058.
DOI: 10.12086/oee.2018.180058