On-Orbit Calibration of Danuri/PolCam. II. Radiometric Calibration
Danuri, South Korea's first lunar orbiter, was launched on August 5, 2022, and has successfully operated 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 comprehensive radiometric calibration pipeline for PolCam's on-orbit data, consisting of dark current removal, smear correction, and flat-fielding. Notably, PolCam's raw data exhibit severe smear artifacts induced by the frame-transfer CCD architecture, which significantly degrade both radiometric fidelity and the accuracy of polarimetric measurements. These smear artifacts have been effectively 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 (DoLP). Finally, we present representative examples of polarimetric measurements to validate calibration performance. Although the current calibration focuses on restoring data quality for qualitative scientific analysis, these results clearly demonstrate the expected inverse relationship between intensity and polarization. The absolute photometric calibration required for quantitative DoLP analysis is reserved for a subsequent publication.