Photometric Correction of Images of Visible and Near-Infrared Bands from Chandrayaan-1 Hyper-Spectral Imager (HySI) HySI Photometric Correction

Photometric correction is a necessary step in planetary image pre-processing since the images of planetary surfaces are acquired by orbiting spacecraft at various observational geometries. In this study, visible (748 nm) and near-infrared (948 nm) bands of Hyper Spectral Imager (HySI) onboard Chandrayaan-1 have been used to derive a preliminary photometric correction for lunar data. The purpose of the proposed photometric correction for HySI is to convert observations taken at solar incidence (i), sensor emission (e), and the solar phase angles (alpha) to a fixed geometry by applying i = alpha = 30 degrees and e = 0 degrees to each image. The Lommel-Seeliger function was used to model the lunar limb darkening effect, while topography data from the merged Digital Elevation Model of Lunar Reconnaissance Orbiter-Lunar Orbiter Laser Altimeter (LRO-LOLA) and SELENE Terrain Camera (TC) was used to correct local topographic effects. Data from Moon Mineralogy Mapper (M-3), SELENE Multiband Imager (MI) and Clementine Ultraviolet and Visible Camera (UV/VIS) were also used to compare radiance, reflectance and phase functions derived from HySI. Our analysis reveals that HySI is darker than M-3 primarily due to low surface radiance conditions observed by HySI. The derived phase functions for the two HySI bands indicate a good correlation between the derived reflectance and phase angle as well as with the phase functions derived for the empirically corrected M-3 data. This approach led to the derivation of a photometric correction for maria regions. Finally, it is expected that the proposed correction would be applicable to all HySI images covering the lunar mare region.

Automated summary

In this study, a preliminary photometric correction method for lunar data using visible and near-infrared bands of HySI onboard Chandrayaan-1 was proposed. The correction involves converting observations to a fixed geometry by applying specific angles and correcting for local topographic effects. The derived photometric correction was found to be applicable to lunar mare regions.