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摘要:
中国1.8 m太阳望远镜(Chinese large solar telescope, CLST)致力于对太阳偏振的高精度及高灵敏度测量。然而其系统本身会引入仪器偏振, 并且在望远镜运行的过程中, 仪器偏振会随其指向的变化而变化。这就降低了系统的测量精度。因此, 需要一个偏振标定单元对其仪器偏振进行标定。为此, 本文对偏振标定的原理和方法进行了研究, 并且给出了针对CLST的偏振标定单元设计方案。
Abstract:It is the main task for the Chinese large solar telescope (CLST) with a 1.8 m aperture to measure the solar polarization with a high accuracy and sensitivity.However, the telescope system itself will introduce instrumental polarization.It also will change constantly with the rotating of the telescope and will reduce the accuracy.Therefore, a calibration unit is necessary to calibrate it.In this paper, we introduced the polarization calibration method and proposed a calibration unit structure.
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Key words:
- solar telescope /
- polarimetry /
- calibration /
- instrumental polarization
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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.
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表 1 CLST系统参数指标
Table 1. The parameters of CLST system
Parameter Value Aperture size of M1/mm Φ1800 Field of view (FOV)/(′) ≥Φ3 Wavelength cover/nm 380~2500 Image space F# F/54.5 Focal length/m ~96 
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表 2 偏振调制方案
Table 2. Polarization modulation program
Number Polarizer angle/(°) λ/4 wave plate angle Stokes 1 0 none [1 1 0 0]T 2 45 none [1 0 1 0]T 3 90 none [1 -1 0 0]T 4 0 45° [1 0 0 1]T
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表 3 偏振标定单元精度要求
Table 3. Accuracy requirement of polarization calibration unit
Parameter Error κ < 0.001 θ1/(°) < 0.05 δ/(°) < 0.1 θ2/(°) < 0.05
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表 4 F2焦点系统参数
Table 4. System parameters in F2
Parameter Value Circular diameter/mm 27 Image scale/(″/mm) 18.93 Focal length/mm 10897 F ratio 6.2 Power density/(W/cm2) 26 Power/W 160
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表 5 驱动电机性能要求
Table 5. Requirements for motor drives
Drive A Drive A1 Drive B Drive B1 Position precision/(°) < 0.05 < 0.05 < 0.05 < 0.05 Position repeatability/(°) < 0.1 < 0.1 < 0.1 < 0.1 Rotation speed/(rpm) < 3 >10 >3 >10
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表 6 转盘各通光孔装置及应用
Table 6. The device and their useful on the rotator
Device Application A1 Rotating polarizer 1 To introduce linear polarization In visible wavelength A2 Rotating polarizer 2 To introduce linear polarization In near infrared wavelength A3 Full FOV A4 Fiber adapter Holder for a fiber A5 Empty For later use A6 Empty For later use B1 λ/4 wave plate 1 To introduce circular polarization In visible wavelength B2 λ/4 wave plate 1 To introduce circular polarization In near infrared wavelength B3 Full FOV B4 Pinhole To align the wave front sensor and so on B5 Resolution target Used for focusing M3 B6 Slanted edge target To measure directly the MTF
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