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

Abstract

Before being used for scientific research, images obtained by solar telescopes with digital detectors 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 are named the White-light Solar Telescope (WST) and the Solar Disk Imager (SDI). The WST and the SDI use Complementary Metal-Oxide-Semiconductor (CMOS) as image sensors, and the main feature of the flat field is the stripe structure which is caused by laser annealing. When used in the ultraviolet wavelengths, CMOS sensors will be 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 the WST and the SDI in the in-flight flat field calibration since the launch of the ASO-S mission on 2022 October 9, the computed flat field images and their evolution during the period. We also briefly present the degradation and the radiation damage of the detectors in the WST and the SDI over time.