Citation: | 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 |
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Overview: Practically all solar phenomena are more or less relative to the solar magnetic field. It produces relativelystable 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 itcan 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 outputin 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 have being developed (such as DKIST, EST), and theStokes polarimetry is their most important observational device for determining the magnetic field. The Chinese largesolar telescope (CLST) with a 1.8-m aperture is a classic Gregorian configuration telescope with an alt-azimuth mount.It will be the second largest solar telescope in the world for a long time. And it is the main task for the Chinese largesolar telescope (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 also will 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.
In this paper, we 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 unitbecause the components before it do not contribute to the Muller matrix. F2 is an extremely useful position in CLST. Itshould be used not only for polarimetric purposes but also for internal alignment. Thus the calibration unit becomes amore versatile device. Then we design the structure of CLST calibration unit with two turnplates. The calibration unitoptics equipped on these turnplates will consist of a rotatable linear polarizer, and two rotatable achromatic quarter waveplates which are in use alternatively for visual range or infrared observations. And there are also some other space on theturnplates to realize the internal alignment purposes.
Optical scheme of CLST
Optical scheme of polarization calibration method
The place of calibration unit in CLST
Structure diagram of CLST calibration rotator
Configuration diagram of CLST calibration unit