Calibration Method Research For Time Divided Polarimetric Imaging System

Solar eruptions,such as coronal mass ejections and flares,accompanied by magnetized plasma,have violent disturbance in space weather and even cause casualties.Therefore,it is an important method for monitoring the space weather to observe and forecast solar activities.With the change of regional density before the coronal mass ejections,the physical information such as electronic density and temperature in the corona can be obtained from the measurements of coronal polarized brightness at700 nm.The solar eruption can be early warned to prevent the destructive influence on mankind lives.There are several approaches for polarimetric detection: division of time,division of amplitude,division of aperture,division of focal plane,etc.One commonly used approach is to rotate polarization elements in front of the camera system,that is division of time.The target cannot change a lot to avoid the unclear polarization image.The imaging optical system having residual polarization effect changes the measured incident polarization states,which influence the polarization accuracy.In addition,the extinction ratio of polarizer,angle error and noise of detector influence the light intensity information from detector,which reduce the polarization accuracy.Therefore,the polarization calibration which can correct the system errors such as extinction ratio and optical system must be carried out in the time-divided polarimetric imaging system.In this paper,the following research work is carried out in view of the demand of polarization calibration in the time-divided polarimetric imaging system:For the time-divided polarimetric imaging system,we investigate the influence of polarization angle error,systematic polarization effect,etc.by using Stokes-Mueller matrix.The pseudo inverse analysis method based on the Kronecker product is used to analyze the influence of Gaussian and Poisson noise on precision.The optimal configurations are proposed for natural light: estimation precision of the calibrated Mueller matrix depends only on the reflectivity of the system,and not on its polarimetric properties.The optimal configurations for different incident polarization states are proposed and it solves the problem that the incident polarization states from collimators influence accuracy.On the basis of studying accuracy of polarization calibration in the presence of Gaussian and Poisson noise,the estimation variance expressions for response matrix are derived by first-order Taylor expansion of the mean square error,and the calibration precision for normalized response matrix is calculated with two types of noise.The influence of source intensity,noise and calibration scheme on accuracy are analyzed.The method to choose the minimum digital number value of the detector,the number of generator and analyzer is proposed,and it can reduce the influence of repeated experiments and time-dependent errors.The theory is verified by Monte Carlo simulations and experiments.The results show that the relative differences between theoretical and experimental results are less 7.5%,and the method can provide theoretical support for the design of polarization calibration scheme in laboratory and orbit.Based on the optimal configurations,the polarization calibration for coronagraph is carried out.The results show that the calibration scheme can obtain the response matrix.The polarization accuracy is better than 1.5%,and the calibration accuracy is better than 0.73%.The design methods of polarizer and polarization detection system are investigated in extreme ultraviolet,and multilayer reflective polarizer is designed at 9.4nm.Two reflective polarizers,polarization generator and analysis system are developed in ultraviolet,in addition,the parameters reflectivity and extinction ratio are measured.The experimental detection and calibration of ultraviolet polarization system are carried out,which can verify the theoretical design.The results show that the extinction ratio of two reflective polarizers is 1:384 at central wavelength and the polarization accuracy is better than 3.1%,and the calibration accuracy is 0.18%,which provides a basis for the follow-up design of extreme ultraviolet polarization system at 9.4nm.