Spectral lunar irradiance measurement campaign at IARC

A field campaign was conducted at Izaña Observatory between April and September 2022 in the framework of the ESA project “Improving the lunar irradiance model of ESA”. This project, launched in 2021, was assigned to a consortium of three members: University of Valladolid/Izaña Atmospheric Reseach Center, National Physical Laboratory (NPL, London) and Flemish Institute for Technological Research (Vlaamse Instelling voor Technologisch Onderzoek, Belgium). The main goal of the project is to use a wider set of ground-based lunar measurements performed with a CE318-TP9 photometer at Izaña and Teide Peak to improve the current Lunar Iradiance Model of ESA (LIME), providing improved spectral interpolation algorithm and uncertainty analysis. This will also provide users in the lunar calibration community a Git hosted python module and GUI to perform lunar calibration using LIME.

Figure 1.- Deployment of the ASD spectrometer in Izaña Observatory during the ESA campaign by University of Valladolid and IARC members in April 2022.

In this project, modifications to the ASD FieldSpec spectroradiometer are proposed, including new fore optics, designed to define the field of view over the full spectral range and a scrambler to ensure all the ASD fibres view the full lunar disk. Additionally, a field reference source with dual apertures is proposed for calibration and performance verification. To do that, the ASD was mounted on a lunar tracker ensuring the ASD fore optics to be centred on the lunar disk. The University of Valladolid team was in charge of performing ASD hyperspectral measurements of the lunar disk at Izaña Observatory for a sufficient number of lunar phase angles (at least three lunar cycles) to capture the possible spectral variability of the lunar disk reflectance with phase angles and libration angles (Figure 1).