| With the development of astronomy and astrophysics,space telescope has become an important observation tool.One of the key research directions of space telescope is radiometric calibration technology.Radiometric calibration is used to correct the non-uniformity of photoelectric response of astronomical telescope and establish the quantitative relationship between the radiation of astronomical target and the output of telescope.Calibraion is the premise that the astronomical data observed by astronomical telescope can be effectively interpreted and quantitatively analyzed.Calibraion is also the guarantee that the astronomical telescope achieves the established scientific goals in the detection depth,photometric accuracy and photometric system deviation.However,the existing radiation calibration scheme,calibration equipment and calibration accuracy of space telescope in China can not meet the needs of space telescope.Therefore,from the application characteristics and calibration requirements of space astronomical telescope,it is of great significance to study the radiation calibration technology of space astronomical telescope.Based on the scientific target and optical mechanical parameters of SVOM VT,the radiometric calibration technology of space telescope is studied.Firstly,the radiometric calibration model of space astronomical telescope is established,and the difficulties that the existing calibration system,brightness of calibration light source and radiometric measurement accuracy can not meet the calibration requirements of space astronomical telescope are analyzed;aiming at the problems that large aperture and extremely low brightness uniform light source are difficult to achieve,and extremely low irradiance is difficult to accurately measure,the whole process cross validation of phased calibration is proposed The scheme includes three calibration stages:detector,whole machine and on orbit.Secondly,in the laboratory detector calibration stage,the detector calibration and characterization platform including high stability light source of plasma lamp,monochromatic light source,integrating sphere,photodiode and picoammeter,vacuum tank refrigeration module and other equipment is designed.The bias field calibration,dark field calibration,flat field calibration and gain calibration,quantum efficiency calibration,readout noise characterization,linearity characterization,dark current characterization,full well capacity characterization,photon response nonuniformity characterization,charge transfer efficiency characterization are carried out.The uncertainty of flat field calibration,gain calibration and quantum efficiency calibration is 0.508%,1.4%and 1.78%,respectively.Then,in the whole machine calibration stage,the verification method of radiometric calibration accuracy based on field observation is studied.The results show that the photometric system deviation of VT to Landolt photometric standard star is 2.48%in blue channel and 3.3%in red channel,which further verifies the accuracy of laboratory calibration results.Besides.the analysis method of on orbit detection limit of space telescope is also studied,and the calculation results shows that the on orbit detection ability of VT can meet the scientific target of 22.5Mv.Finally,in the on orbit calibration stage,based on the structural characteristics of VT,the on orbit calibration system based on LED ring and diffuse reflector is designed,and the simulation and experimental verification are completed.The measured results show that the flat field accuracy between adjacent pixels is better than 0.5%.The dissertation takes the radiometric calibration technology of space astronomical optical telescope as the research object,and carries out the whole process cross validation radiometric calibration principle analysis,scheme design,experimental verification and accuracy analysis.The main work and innovation of this paper are as follows:(1)The radiometric calibration model of space astronomical telescope is established.Aiming at the problem that it is difficult to realize the large aperture and extremely low brightness uniform light source,and it is difficult to accurately measure the extremely low irradiance in the calibration requirements of space astronomical telescope,the overall calibration scheme of cross validation in the whole process of phased calibration is proposed,which includes three calibration stages of detector,whole machine and on orbit.(2)The radiometric calibration model of space telescope CCD detector is established.Compared with the traditional CCD radiometric calibration model,the relative calibration is divided into bias calibration,dark field calibration and flat field calibration,and the absolute calibration model is divided into gain calibration and absolute quantum efficiency calibration.(3)In dark current characterization,in order to solve the problem of the low efficiency of low frame rate CCD dark current temperature characteristic test,the dark current accumulation characteristic of frame transfer CCD in the process of transfer readout is analyzed,and the dark field image gradient linear fitting method is proposed to quickly calculate the dark current.This method does not need to collect 0s bias image and long integration time dark field image at each temperature,only needs a 1s integration time dark field image,and does not need to wait for the temperature of the temperature controller to be stable.Taking the dark current test of-80?10?sampling interval of 2?as an example,the test time can be shortened from 1 day to less than 1hour;In the charge transfer efficiency characterization,in order to solve the problem of insufficient application conditions of traditional extended edge pixel method,the relationship between the flat field image variance distribution and the charge transfer efficiency is analyzed by using equivalent infinitesimal,and the flat field variance linear fitting method is proposed to test the charge transfer efficiency.This method makes full use of the gray data of the whole flat field image,and the measurement accuracy can reach 10-6.(4)In the process of CCD flat field calibration,in order to solve the problem that there is no light source close to the gamma ray exposure spectrum,a relative calibration method of flat field correction matrix under the gamma ray exposure spectrum synthesized by monochromatic flat field image is proposed.In the system gain calibration,the charge sharing phenomenon of back illuminated CCD leads to the big problems of traditional gain test methods such as signal-variance method and photon transfer curve method.In order to solve this problem,the charge sharing model is established by using random variable numerical simulation method,and the quadratic fitting formula of photon transfer curve with charge diffusion factor as parameter is proposed.The experimental results show that this method can achieve 1.4%uncertainty of gain calibration;In CCD absolute quantum efficiency calibration,a two-step calibration method is proposed to solve the problem that it is difficult to accurately measure the illumination source with extremely low irradiance.The method consists of two photodiodes mounted on CCD position and integrating sphere respectively.The first step is to calibrate the proportional relationship between the two photodiodes in high illumination condition.The second step is to calibrate the CCD by integrating sphere photodiode in low illumination condition,and convert the measured illumination data to CCD position.The experimental results show that the uncertainty of absolute quantum efficiency calibration can reach 1.78%.(5)In order to solve the problem of excessive dark current in red band caused by insufficient cooling temperature in the process of field observation experiment,an image preprocessing method of adding dark current gradient field calibration to the traditional methods of bias calibration,dark field calibration and flat field calibration is proposed.This method can eliminate the dark current accumulated in the process of CCD transfer readout at low frame rate.Typical test results show that the background uniformity can be improved from 10.94%to 1.32%.(6)Based on the structural characteristics of VT telescope,an on orbit calibration system based on LED ring and diffuse reflectors is designed.The diffuse reflectors are installed on the back of VT camera shutter,which has the advantages of high integration,light and miniaturization.The measured results show that the flat field accuracy between adjacent pixels is better than 0.5%,which can meet the on orbit calibration requirements of space telescope. |
PDF Full Text Request