Research On Flat Field Measurement Method For Solar Telescope Based On Diffuser

The occurrence of catastrophic space environmental events may threaten the safety of spacecraft in orbit,affect the accuracy of communication and navigation,and even affect ground power systems and oil pipelines.Using large field of view solar telescope and coronagraph observation can monitor the activity and process of solar eruption from a global perspective,which is of great significance to space environment prediction and solar physics research.Flat field is used to describe the non-uniformity of the whole telescope system,which is a necessary step in scientific data processing.The effect of flat field correction determines the accuracy of subsequent data calibration processing,and has an important impact on the quality of data products.At present,the flat-field correction methods of large-field solar telescope has problems such as large amount of calculation,complicated algorithm,and restricted by weather and time.Therefore,this article focuses on this issue,based on diffuser materials to carry out related research work,and explored a new set of solar telescope flat field measurement and correction methods.The diffusers can spread the incident non-uniform sunlight into a specific surface light source with known intensity distribution,which provides a new technical way for the flat field measurement of full disk solar telescope and coronagraph.In this paper,the flat field measurement method of solar telescope based on diffusers is firstly carried out in China.We have systematically studied the application of four kinds of diffusers-opalescent glass,ground glass,Gaussian type diffuser and engineering diffuser in coronagraph and full disk solar telescope,and carried out relevant simulation,experimental verification and data analysis.The main results of this paper are as follows:1.The surface light source formed by opal glass is close to the ideal uniform surface light source,and the transmittance is close to the coronal brightness.It is suitable for coronagraph.Flat field image can be obtained by direct shooting.Compared with the existing coronameter flat field measurement method,this method has higher measurement accuracy,makes the flat field measurement no longer subject to the weather and time constraints,and allows the opal glass to have a certain adjustment error,which is convenient for engineering application.2.The uniformity of the surface light source formed by ground glass in the solar field of view is 99% and the transmittance is about 0.2%.If the exposure condition of the telescope is allowed,the flat field image can be obtained directly.Through cross calibration with Gong and HMI data,the corrected data indicators are significantly improved,and the correlation between low-frequency profile and theoretical edge darkening reaches 0.99.3.The diffusion angle of Gaussian scatterers is further reduced,and the field transmittance is about 20%,which shortens the exposure time difference between the flat field measurement and the conventional observation.After background correction,the uniformity of diffuse surface light source can reach 99.6%.Through the experimental verification and data analysis in the heliospheric and chromospheric telescopes,the correction accuracy of the chromosphere local field of view reaches 0.5%,and can effectively correct the large-scale contour nonuniformity of the whole solar surface.4.The diffusion shape of the engineering scatterers to the light source is flat top type,so the light source of the diffusion surface has good uniformity,and the transmittance is equivalent to that of the Gaussian scatterer.The flat field measurement and correction can be completed quickly and conveniently.On this basis,the whole day velocity field calibration is carried out.The correlation coefficient between the whole solar velocity field and HMI velocity field is 0.923,and that on the equator is 0.958.Compared with the images before and after correction,it can be found that the proposed method can effectively correct the influence of small-scale interference such as dirty spots,dust,stripes,etc.At the same time,the large-scale distortion such as uneven radial intensity attenuation in different directions can also be corrected or corrected,so that the change of image intensity is consistent with the edge darkening.It is not affected by the whole process of correction.Through cross validation with international data or self verification of observation equipment data,the accuracy of leveling correction is quantitatively evaluated by using different indexes,which proves that the method in this paper has good correction effect.The flat field measurement method in this paper constructs a surface light source with similar intensity to the conventional observation target,so the flat field image can be obtained without moving the sun image.This series of methods make the flat field measurement of the coronameter no longer subject to the constraints of time and weather,but also makes the flat field measurement of the omni-directional telescope change from fitting calculation to direct shooting.The flat field correction algorithm is simple,universal,low control precision and easy to realize automation.It provides a new technical method for the flat field measurement of the existing and future space-based large field solar observation equipment in China.