Flat-fielding of Full-disk Solar Images with a Gaussian-type Diffuser

Full-disk solar chromospheric and photospheric images are widely used to monitor solar activity. Flat-fielding is a basic step during scientific data production. Here, a Gaussian-type diffuser is used to measure the flat-field of full-disk solar chromospheric and photospheric images. The effectiveness of the method is verified by theoretical simulation and practical measurement. First, the uniformity of the light source generated by the Gaussian diffuser and the sunlight, with a field of view the same as the full-disk solar image, is calculated. The full width at half maximum (FWHM) of the scatter angle of the Gaussian diffuser is one degree, such that the light source presents a Gaussian distribution. After the correction of the Gaussian background, the uniformity of the area light source is 99.6%, which can be regarded as a uniform area light source. Second, experiments are conducted on full-disk solar telescopes at Huairou solar observing station and Yangbajing station. The diffuser covers the entire entrance aperture when taking flat-field frames. Through qualitative and quantitative analysis of experimental results, it is confirmed that a revised flat-field frame can modify uniformity caused by various aspects of the telescope, including vignetting, non-uniformities of filters, and contamination of optical components.