The Study On X-ray Focusing Mirror Of Follow-up X-ray Telescope On Board Einstein Probe

Woter-I type grazing incidence X-ray focusing mirror has been widely used in X-ray astronomical observation because of it's high positioning accuracy and high signal-to noise ratio.It can adopt multi-layer structure,which can greatly improve the effective area and realize high-precision observation.Einstein probe(EP)is anastronomical exploration satellite program proposed by the Chinese Academy of Sciences in 2013.In order to achieve high-precision positioning,high-quality imaging and obtain observation data with high signal-to-noise ratio,The Follow-up X-ray telescope(FXT)which is one of the main payloads onboard EP adopts a multi-layer nested Wolter-I X-ray focusing mirror system as its optical system.However,there is relatively little research work on wolter-I and other x-ray focusing optical systems in China.In order to further study the x-ray focusing optics,the Institute of High Energy Physics of Chinese Academy of Sciences and its cooperative units actively carry out the domestic independent research and development of the focusing mirror,and provide the domestic design scheme for FXT.This paper aims to propose the domestic design of Wolter-I X-ray focusing mirrors,establish a complete set of performance simulation software,and simulate the performance of focusing mirrors in detail.Meanwhile,in order to establish the connection between the actual performance and machining tolerance and provide reference for the production and test of focusing mirror,a simulation algorithm under actual surface condition is completed.The algorithm is proved to be very effective in the application of various kinds of deformation data.Specific work of this paper mainly includes the following points:(1).Based on the principle,the key parameters of multi-layer nested structure of focusing mirrors are quantitatively analyzed.According to the optical performance indexes of FXT optics and domestic focusing mirror,the optical design of FXT domestic focusing mirror is completed.A complete set of mirror simulation software is designed independently.Based on ray tracing and Monte Carlo simulation algorithm,the software can simulate the performance of mirrors,including effective area,FOV,vignetting,influence of shading filter and stray light distribution.The simulation results show that the effective area of domestic design is 463.7±0.2 cm~2@1.25 ke V,which can well meet the observation requirements of FXT.(2).The surface figure error(low frequency error)and roughness error(high frequency error)of the focusing mirror are simulated.This paper introduces two commonly used surface figure error simulation methods:one is to intervene the normal vector of the reflecting surface by using the disturbance of mathematical form,and the other is to reconstruct the three-dimensional model of the surface by using the measured data of surface shape and simulate according to the finite element algorithm.The cross-validation results of the two methods show that the finite element algorithm can well establish the relationship between the optical properties and the machining tolerance.(3).The assembly and equipment of focusing mirrors are briefly introduced.The optical performance tests of domestic focusing mirror sample and structural thermal controls module provided by ESA are completed.At the same time,the simulation method of surface error is verified by using the test results of structural thermal control module.(4).In order to meet the requirements of focusing mirror performance test,this chapter introduces a design of X-ray secondary multi-target source.The source has high monochromaticity and simple and fast operation.At present,it has been successfully applied to the mirror test and electronic joint debugging of EP-FXT detector pn CCD.(5).A calibration method of pn CCD's energy response matrix is introduced.The method is verified by the calibration data of CCD236.The spectrum fitting residual of the energy response matrix generated by this method can be controlled within 3?and the fitting deviation of the line center of the peaks is less than 10 ev.The innovations of this paper can be summarized as follows:(1).According to the optical performance index of FXT and the optical performance index of domestic focusing mirror,a set of domestic mirrors design is proposed.(2).A complete set of focusing mirror simulation software is designed independently.Based on ray tracing and Monte Carlo simulation algorithm,the software can simulate the performance of focusing mirrors,including effective area,field of view and vignetting,the influence of shading film and stray light distribution and it is of great significance for the domestic development of focus mirrors.(3).Based on the simulation algorithm of ideal mirror,two methods to simulate the real surface state are studied,which are the disturbance simulation of mathematical model and the simulation of finite element using measured surface data.The simulation algorithm can complete the prediction of the optical performance of the mirrors and convert the optical performance into measurable mechanical tolerance,which provides an important reference for the production and testing of the focus mirrors in the future.(4).Using the performance test equipment,the pre-calibration of FXT domestic focusing mirror sample and structural thermal controls module provided by ESA is completed for the first time,the analysis and research of test data are carried out.