A New Approach for Correcting Distortions in Full-Disk Solar Images

Due to low-quality flatfields used for corrections or the vignetting effects of telescopes, some full-disk solar images show large-scale non-radial distortions.In this paper we propose a processing procedure to fix this problem.The steps are (1) calculating the location of the center of the sun in a raw image, (2) transforming the raw image with the full-disk centered under the Cartesian coordinates to that under the polar coordinates, (3) calculating the median of the pixel values at each radius from the solar center in the raw image, which yields an average radial profile, (4) fitting the radial profile with a high-order polynomial function, (5) removing the radial profile from the raw image resulting in a residual image, (6) creating a model of the distortion pattern for correction by smoothing the residual image in (5) with a large-scale median filter, and (7) producing the final corrected image by a standard procedure.It is apparent that the key factor for the correction is the value of the window width of the median filter.According to our experience a value of the window width between 2% to 5% of the solar angular diameter is suitable for the correction of a full-disk solar image in most cases.We have processed several distorted images and show the results in the paper.This procedure should effectively repair distortions of full-disk solar images and significantly improve image quality.We discuss some drawback of this approach in the paper as well.