畸变全日面观测图像的修复

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

欠佳的平场改正或者不理想的光路会导致全日面观测图像的畸变。针对这一问题,提出了一种解决方案,并给出了详细的算法描述。该算法的核心是扣除一个均匀的径向轮廓曲线,然后在残差图像上进行大尺度中值滤波以获得光滑的改正曲面。通过这一算法,可以极大改善全日面图像的畸变,提高图像质量。最后还对该方法的局限性进行了讨论。

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.