TANAGER: Design and Validation of an Automated Spectrogoniometer for
Bidirectional Reflectance Studies of Natural Rock Surfaces
Abstract
Laboratory measurements of reflectance spectra of rocks and minerals at
multiple viewing geometries are important for interpreting spacecraft
data of planetary surfaces. However, efficiently acquiring such
measurements is challenging, as it requires a custom goniometer that can
accommodate multiple, bulky samples beneath a movable light source and
detector. Most spectrogoniometric laboratory work to date has focused on
mineral mixtures and particulates, yet it is also critical to
characterize natural rock surfaces to understand the influence of
texture and alteration. We designed the Three-Axis N-sample Automated
Goniometer for Evaluating Reflectance (TANAGER) specifically to rapidly
acquire spectra of natural rock surfaces across the full scattering
hemisphere. TANAGER has its light source and the spectrometer’s fiber
optic mounted on motorized rotating and tilting arcs, with a rotating
azimuth stage and six-position sample tray, all of which are fully
motorized and integrated with a Malvern PanAnalytical ASD FieldSpec4
Hi-Res reflectance spectrometer. Using well-characterized color
calibration targets, we have validated the accuracy and repeatability of
TANAGER spectra. We also confirm that the system introduces no
discernable noise or artifacts. All design schematics and control
software for TANAGER are open-source and available for use and
modification by the larger scientific community.