On-Orbit Calibration of Danuri/PolCam. II. Image Artifact Correction

Abstract

Danuri, South Korea’s first lunar orbiter, was launched on 2022 August 5, and has completed its two-year nominal mission phase. The wide-angle Polarimetric Camera (PolCam) onboard Danuri is the first instrument to conduct global polarimetric observations from lunar orbit. This paper presents the image artifact correction pipeline for PolCam’s on-orbit data. It encompasses the primary steps of the radiometric calibration, consisting of dark current removal, smear correction, and flat-fielding, except for inter-channel relative response and the final radiance calculation. Notably, PolCam’s raw data exhibit severe smear artifacts induced by the frame-transfer CCD architecture, which degrade both radiometric fidelity and the accuracy of polarimetric measurements. These smear artifacts have been mitigated through a rigorous correction algorithm, restoring data quality to a level sufficient for scientific analysis and facilitating the precise derivation of the degree of linear polarization. These steps serve as an essential prerequisite for conducting subsequent absolute radiance and photometric calibrations. Finally, we present representative examples of polarimetric measurements to validate the calibration performance. Although the current calibration focuses on restoring data quality for qualitative scientific analysis, these results demonstrate the expected inverse linear relationship between intensity and polarization.