Dither Pattern and Results of Inflight Flat-field Calibration of the Lyman-alpha Solar Telescope onboard ASO-S Mission

Before being used for scientific research, images obtained by solar telescopes with digital detector must be corrected for flat-field, which reflects the non-uniformity coming from the non-uniform transmission efficiency of optical components and response of detector pixels. That means flat-field correction is a necessary step in the pipe-line of high-level scientific data. As one of the three payloads of the Kuafu-1 (Advanced Space-based Solar Observatory: ASO-S, Chinese name Kuafu-1) mission, the Lyman-alpha Solar Telescope (LST) consists of three scientific instruments: including a dual-waveband Solar Corona Imager (SCI), and two full-disk solar telescopes which named the White-light Solar Telescope (WST) and the Solar Disk Imager (SDI). WST and SDI use Complementary Metal-Oxide-Semiconductor (CMOS) as image sensors, and the main feature of the flat field is the stripe structure caused by laser annealing. When used in the ultraviolet wavelengths, CMOS sensors will subject to certain degradation and radiation damage, as well as the pollution of water vapor condensation and organic matter accumulation, which will have some influence on the obtained flat field. This paper mainly presents the dither pattern and its optimization used by WST and SDI in the in-flight flat field calibration since the launch of ASO-S satellite on 9 October 2022, the computed flat field images and their evolution during the period. We also briefly present the dector degradation and radiation damage of WST and SDI.