Flat-field measuring and correction method for full-disk solar image based on ground glass

Abstract

Full-disk solar chromosphere and photosphere images are mainly used for monitoring solar activities. Flat-field correction of observation image is a basic work during data processing. In history, it is difficult to obtain uniform area light whose intensity is similar as sun. So flat-field measurement and correction of full-disk solar image usually depend on moving images and mathematical calculation. In order to overcome some disadvantages of traditional methods, such as large calculating quantity, complicated algorithms and difficult to realize automation, we research and design a new flat-field correction method based on ground glass. In this text, sunlight diffused by ground glass in field of view is used as uniform area light for flat-field measuring. We simulated the area light that sun diffused by ground glass, calculated the uniformity of area light in field of view. The uniformity of area light is more than 99%, so it can be used as uniform area light for flat-field measurement directly. We designed flat-field measurement device and formulate steps and methods for flat-field correction. After numerical stimulation and calculation, experiments carried out on solar chromospheres telescope and photosphere telescope separately. The result shows that the flat field obtained in this text could correct high-frequency information such as spots and non-uniformity of light path. Subjected to telescopes limited ability to eliminate stray light, flat-field measurement and correction also causes low-frequency information anamorphous because the angle of diffused light is much larger than normal observation. In order to amend low-frequency information distortion, we use multi-scale morphology filtering and median filtering to extract the low-frequency image from flat-field corrected image. Then the extracted low-frequency image is amended on the basis of theoretical limb darkening image. After flat-field correction and amendment, the correlation coefficient between corrected image and theoretical image is more than 0.99. At last, photosphere image of HMI and chromospheres image of GONG are used as reference images to quantitatively evaluate corresponding result in various field of view. The preprocessing of reference images includes three steps: (1) Centralization and geometric correction of experiment images and reference images; (2) unify reference images’ pixel resolution based on experiment images; (3) according to fitting intensity attenuation of experiment images’ margin, convolve the same Gaussian profile to unify the atmosphere effect on reference images and experiment images. The evaluation result shows that all indexes of corrected image are better than that of origin images. The flat-field measurement and correction method in this text is simple and convenient, and it is easy to realize automation. We suggest that the above mentioned method can be considered as a new method for full-disk solar telescope flat-field measurement and correction. In the future, we need to use diffuser which output angle is smaller during flat-field measurement to shorten exposure time and narrow the exposure time difference between flat-field measurement and normal observation. If a full-disk solar telescope wants to use this method to measure and correct flat-field, the ability of stray light elimination must be considered and optimized during design stage, and the data processing would be more simple and easy.