Stray light analysis by ray tracing simulation for the wide-angle multiband camera OROCHI onboard the Martian Moons eXploration (MMX) spacecraft

The Martian Moons eXploration (MMX) spacecraft is equipped with two cameras, i.e., the TElescopic Nadir imager for GeOmOrphology (TENGOO) and the Optical Radiometer composed Of CHromatic Imagers (OROCHI), for the scientific observation of the Martian moon Phobos. OROCHI is a wide-angle multiband camera system comprising seven cameras with different bandpass filters and one monochromatic camera in the visible and near-infrared range. Thus, OROCHI can simultaneously obtain multiband images. Previous space observations have revealed that the reflection spectra of Phobos exhibit regional differences, including a red unit and a blue unit with weak redness. Moreover, Phobos shows an absorption band near 650 nm, attributed to the red unit of Phobos. Global observations using OROCHI require the determination of such regional variations with high precision; therefore, we must reduce noise in the optical system design. In this study, to meet this requirement, we focus on stray light reduction. Stray light is a source of noise generated by reflections at surfaces of optical components (e.g., sensors, filters, lenses, and lens barrels). We design simulation models of the OROCHI optical system, measure the reflectance of its charge-coupled device (CCD) image sensor, and conduct stray light analysis via ray tracing. We estimate the ratio of the stray light intensity to the target signal intensity by deriving both the reflected (constituting stray light) and unreflected rays. We show that placing the filter away from the sensor and reducing the reflectances of the filter and lenses effectively reduce stray light. Such approaches are incorporated into the preliminary design of OROCHI.