Research On NVST Solar Image Interference Fringe Removal Method

The activity of the sun is closely related to human life.It constantly transmits energy to our earth all the time.Every tiny irregular activity will have a huge impact on the earth.Therefore,it is important to monitor and predict the sun in real time and study its internal structure.The one-meter new vacuum solar telescope(NVST)is the largest vacuum solar telescope of China.When the telescope collects data through the CCD(Charge-coupled Device),there are occasional thickness fringes caused by film interference,which will reduce the quality of NVST data and interfere with solar physicists' research.In this paper,the interference fringes existing in NVST images are combined with digital image processing techniques to find an effective method to remove interference fringes and improve the quality of NVST data products.Firstly,the characteristics of NVST interference fringes are analyzed,and the shape characteristics and periodicity of interference fringes are studied.The spatial distribution and frequency distribution characteristics of interference fringes are studied by spatial domain and frequency domain filtering.The median,Gaussian,and averaging filtering processes are used in the spatial domain.The frequency domain is first converted to the frequency domain image by Fast Fourier Transform(FFT).The frequency domain Bottworth and the frequency domain Gaussian filtering method are used to study the fringe extraction effect.Then,based on the multi-scale wavelet transform,the NVST image is decomposed into multiple scale images,and the frequency domain filtering method is used to remove the interference fringes in the scale image.In addition,the extraction method of the fringe flat field is also studied.By superimposing multiple scale images with fringes to form the fringe flat field,it can be used for the interference fringe flat field in the simulation experiment.Next,the area detection of the interference fringes is studied to further improve the interference fringe removal scheme.In the scale image with fringes,the Radon transform is used to enhance the characteristics of the interference fringes and the adaptive partitioning method is used to search for the block images containing the fringes,and the frequency domain Gaussian filter is used to filter the interference fringes on each of the block images.After the original image processed using the adaptive partitioning method,the interference fringes in the image are substantially removed.Finally,the effectiveness of the method is verified by simulation experiments and experiments using real NVST data.The fringe flat field extracted by the superposition method and the fringe-free NVST image are used to construct the simulation data,and the interference fringes are removed by using the multi-scale wavelet transform and the adaptive partitioning method studied in this paper.The removal effect of the method is verified by standard error,peak signal-to-noise ratio and structural similarity index image evaluation methods,and the effectiveness of the fringe removal method proposed in this paper is proved.Then the fringe removal experiment of the method is carried out on the real data of NVST.The results show that the proposed interference fringe removal scheme is reasonable and effective for real NVST data.