Polarimetric Technology For Ground-based Large Solar Telescope

Most of the solar phenomena are more or less relative to the solar magnetic field.So the knowledge of its magnitude and direction is crucial for studying the physical mechanism of the corona heating,flare’s trigger and coronal mass ejections,so as to realize the disastrous space weather warning.The solar magnetic field can be measured usually by a polarimetry at some special spectral lines,which should be sensitive to the Zeeman effect.To achieve these targets,it is required to measure the solar polarization with a high accuracy and sensitivity.However,the processing and assembling errors of the polarimeter and the instrumental polarization introduced by the telescope system itself will reduce the polarization measurement accuracy.Therefore,taking Chinese Large Solar Telescope（CLST）for example,we carry out theoretic and applied researches on polarimetry,calibration and instrumental polarization of the ground-based large solar telescope.Firstly,the polarimetric technology is studied.The influences of modulation matrixand intensity measurement error are analyzed,and the requirements are given.What’s more,the measurement error induced by source intensity scintillation for single-pass imaging polarimetries is analyzed,and a calculation method based on double-pass is proposed to eliminating this effect.The emulational and experimental results validate the feasibility.Secondly,a compact simultaneous full-Stokes vector polarimeter based on micro-retarder arrays is proposed.We have validated the feasibility of the proposed method,and a feasible example is also given.To verify the proposed method,we also simulated the polarimeter using the software ASAP（Advanced System Analysis Program）.And the result conforms to our analysis.Thirdly,the polarimetric calibration technology is studied.The influences of the polarizer and wave plate processing and assembling error are analyzed,and the requirements are given.What’s more,a calibration experiment has done.And the calibration accuracy reaches to 3×10^-3.Fourthly,a dynamic calibration technique with modeling the instrumental polarization as a function of the time and season based on the telescope location is proposed.What’s more,two optical compensation programs based on seven mirrors and three mirrors are proposed.The experiments of measuring the Muller matrix of the mirrors coated with aluminum have done.And the results confirm our analysis.Fifthly,a polarization calibration unit for 1.8m Chinese Large Solar Telescope is design.The unit is located behind the second focus,and can be used to calibrate the other system parameters.In this paper,a deep theoretical and applied research is carried out about the polarimetric technology for ground-based large solar telescope.The detailed solutions for the theoretical and engineered problems involved in the polarimetry,calibration and instrumental polarization are presented.At the same times,the complete technology roadmap for the polarimetry system of the CLST is also provided.