| 其他摘要 | The accurate measurement of the solar atmospheric vector magnetic field is important for the analysis of the generation and connection of various violent activities in the solar atmosphere. At present, the precise measurement of the magnetic field is mainly based on the high-precision polarimetry of magneto-sensitive lines caused by Zeeman effect and magneto-optical effect. In order to obtain high-precision magnetic field information, solar physicists demand high precision in the polarimetry of the telescope. Nowadays, the polarimeric accuracy of the current mainstream solar optical telescopes is 10−3Ic ~ 10−4Ic . If the polarimetric accuracy wants to be improved further, the polarimetric errors that may be introduced by each subsystem of the telescope need to be analyzed and suppressed more finely.Fiber Array Solar Optical Telescope of the second generation (FASOT-2), which is being developed by Yunnan Observatories, Chinese Academy of Sciences, aims to simultaneously measure polarization signals of two-dimensional of the target source and that of spectral lines at different atmospheric depths. The polarimetric accuracy of FASOT-2 is required to reach 8 × 10−4𝐼𝑐 . In order to achieve the scientific observation target of FASOT-2 telescope with high polarimetric accuracy over broad band, the polarimetric errors that may be introduced by each subsystem of FASOT-2 telescope need to be carefully calculated and suppressed. As one of the core terminal devices of the solar optical telescope to measure the magnetic field, it is necessary to analyze the polarimetric performance of the polarimeter in detail. in this paper, the three studies have been carried out on the polarimetric system of FASOT-2 telescope.1. A dual-beam polarimeter with high polarimetric efficiency over the wide band has been designed, which can meet the scientific goal of the FASOT-2 telescope for high-precision measurements of target magneto-sensitive lines over 500nm∼900nm. The polarimetric efficiency satisfies 𝜖𝑄𝑖𝑛/𝐼𝑖𝑛 ≥ 0.5, 𝜖𝑈𝑖𝑛/𝐼𝑖𝑛 ≥ 0.51, 𝜖𝑉𝑖𝑛/𝐼𝑖𝑛 ≥ 0.52, 𝜖𝑡𝑜𝑙 ≥ 0.98, and the efficiency balance among them is also well guaranteed. In addition, the polychromatic characteristic of the polarimeier is achieved by using non-achromatic waveplates as the modulator, which avoids the defect, sensitive to ambient temperature or applied voltage, of liquid crystal materials (FLC, LCVR). And the stability of waveplates is higher, but the time resolution is lower than that of liquid crystal materials. The non-achromatic waveplates also avoids the defect that the retardance of the super-achromatic waveplates over the wide band is not accurate. Furthmore, the non-achromatic waveplates can be more available in the market. The polarimeter designed in this paper has the features of simple structure, high stability, strong anti-interference and simple maintenance. Moreover, it adopts the modulation method of Reduced Polarimetric Optics Switching(RPOS), which can minimize the polarimetric errors introduced by the telluric Seeing and different gain factors along different beam paths at the same time. In addition, the polarimeter is combined with the calibration unit to minimize the additional polarimetric errors introduced by the telescope itself. Thus the polarimetric accuracy of the telescope can be improved further.2. The parameters such as the retardance 𝛿 and fast-axis azimuth 𝜃 of the non-achromatic waveplates that constitute the wide-band polarimeter and the polarization calibration unit are accurately measured and corrected, which can further minimize the polarimetric errors introduced by the manufacturing defects of optical components. In this paper, a measurement system based on the fitted intensity method and the spectral analysis method is established, which can achieve high accuracy measurement for non-achromatic waveplates with the retardance over 0 ∼ 2𝜋. The measurement accuracy of the waveplate with retardance ≠ 𝜋 is higher than that of the conventional optical intensity method more than one order of magnitude, and the spectral analysis method based on this measurement system can overcome the defect that the optical intensity method cannot measure the waveplate with the retardance of 𝜋. And the measurement accuracy of the waveplates with the retardance near 𝜋 is higher by using the spectral analysis method. The measurement system can be used to accurately measure and calibrate the the retardances of the non-achromatic half waveplates and quarter waveplates, a part of polychromatic polarimeter.3. The polarimetric efficiency of the assembled polychromatic polarimeter should be actually measured in the laboratory and compared with the theoretical values before it is installed on the telescope. In order to quantitatively measure the polarimetric errors that may be introduced by mounting errors and further analyze the effect of this bias on the performance of the FASOT-2 telescope. The research of the polarimetric efficiency of the polychromatic polarimeter is based on the measurement of the Mueller matrix at a certain wavelength point and a certain modulation state. By repeatedly measuring the Mueller matrix of the polarimeter at different modulation states, the modulation matrix O can be obtained, and then the demodulation matrix D is obtained by calculating the generalized inverse matrix of the O. Finally, according to the definition of the polarimetric efficiency, thus the polarimetric efficiency(𝜖𝑄𝑖𝑛/𝐼𝑖𝑛，𝜖𝑈𝑖𝑛/𝐼𝑖𝑛，𝜖𝑉𝑖𝑛/𝐼𝑖𝑛，𝜖𝑡𝑜𝑙) of polychromatic polarimeter at a particular wavelength point can be obtained. By changing the dispersion center wavelength of the spectrometer, the polarimetric efficiency of the polarimeter at 500nm∼900nm can be measured. The measured results show that the polarimetric efficiency has little deviation from the theoretical value after excluding the errors introduced by other subsystems of the telescope. And the polychromatic polarimeter is not sensitive to the installation errors of optical components. So the polarimetric errors of the telescope are not be introduced by the polarimetric sys tem. Therefore, the source analysis and suppression of polarimetric errors of FASOT-2 telescope may be more focused on other subsystems.In this paper, a dual-beam polychromatic polarimeter of the FASOT-2 telescope with high polarimetric efficiency over the wide band has been designed, and the accurate measurement of the retardance of the waveplates, the optical components used in the polarimetric system, have been completed. In addition, the actual measurement of the polarimetric efficiencies of the polychromatic polarimeter over the broad band have also been completed. The measured results confirm the superior performance of the designed polychromatic polarimeter. This is an important step for the FASOT-2 telescope to achieve high precision polarization measurement over the wide band.Key Words: FASOT-2 telescope, Polychromatic polarimeter, High polarimetric efficiency, Accurate measurement of the retardance, Accurate measurement of Mueller matrix |
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