Calibration of NIR 2 of Spectral Profiler Onboard Kaguya/SELENE

Abstract

The Spectral Profiler (SP) is a visible–near infrared spectrometer onboard the Japanese Selenological and Engineering Explorer (SELENE), which was launched in 2007 and observed the Moon until June 2009. The SP consists of two gratings and three linear-array detectors: VIS (0.5–1.0 $\mu\hbox{m}$ ), NIR 1 (0.9–1.7 $\mu\hbox{m}$ ), and NIR 2 (1.7–2.6 $\mu\hbox{m}$ ). In this paper, we propose a new method for radiometric calibration of NIR 2, specifically for the dark output (background) estimate, which is different from the previous method used for VIS and NIR 1. We show that the reflectance spectra of NIR 2 derived from the new radiometric calibration show less noise than those of the previous method. Based on an analysis of the reflectance spectra at exposure sites of the end-member minerals on the lunar surface, we demonstrated that the spectral features of the 2- $\mu\hbox{m}$ band in the NIR 2 spectra are consistent with those expected from the minerals inferred from the features of the 1- $\mu\hbox{m}$ band in the VIS and NIR 1 spectra. Finally, we examined the repeatability of the radiometric calibration of NIR 2 using the SP data near the Apollo 16 landing site observed at four different times. The typical difference in the reflectance at wavelengths $ was a few percent, which is within the uncertainty due to the error in the background estimate, suggesting that there was no significant change in the sensitivity of NIR 2 over the mission period.