In-flight Radiometric Calibrations of WST and SDI

Abstract

The Ly$\alpha$ Solar Telescope (LST) is one of the payloads on board the Advanced Space-Based Solar Observatory (ASO-S, also known as $Kuafu$-1) mission, which consists of three scientific instruments: a White-light Solar Telescope (WST), a Solar Disk Imager (SDI), and a Solar Corona Imager (SCI). Both WST and SDI have a field of view of 1.2 $R_{\odot }$ ($R_{\odot }$ represents the solar radius and the full field of view spans 38.4${}^{\prime }$) and operate in the 360$\pm$2 nm (near white light) and 121.6$\pm$4.5 nm (i.e., Ly$\alpha$) wavebands, respectively. Using imaging data from WST and SDI, we can explore the dynamics and evolution of solar activities in the lower atmosphere (from the photosphere to transition region), for example, to study the triggering mechanisms of solar flares, the physical properties of white-light flares, and the morphology evolution and kinematics of erupting filaments/prominences, as well as to derive physical parameters of the solar atmosphere. To obtain the physical parameters of different features in the solar atmosphere observed by WST and SDI, such as energy of flares and temperature and density of prominences, it is essential to convert the digital number (DN) of their observations into physical units (e.g., erg $·$ cm${}^{-2}$ $·$ s${}^{-1}$ $·$ sr${}^{-1}$) through a process known as radiometric calibration. Radiometric calibration is a necessary step in the production of scientific data from WST and SDI. Currently, we perform in-flight radiometric calibrations of WST and SDI using the Sun as a reference source. For WST, we utilize solar spectral data released by the American Society of Testing and Materials (ASTM) in 2020, while for SDI we use the Extreme Ultraviolet Sensors (EUVS) aboard the Geostationary Operational Environmental Satellite R (GOES-R). In this paper, we provide the in-flight radiometric calibration factors and their uncertainties for WST and SDI during the normal observation period from August 2023 to February 2024. Furthermore, an empirical expression for the WST in-flight radiometric calibration factors is derived by fitting the daily averages of these coefficients. By utilizing the radiometric calibrated data, we can calculate the energy radiated by solar flares in both the near-white-light and Ly$\alpha$ wavebands and derive the density of prominences. These calculations are critical for achieving the scientific objectives of WST and SDI.