Carruthers Data Processing Pipeline: Radiometric Responsivity Calibration

Abstract

The Carruthers Geocorona Observatory is NASA's first mission dedicated to investigating the fundamental nature of Earth's exosphere and its dynamic response to space weather. Its primary payload, the GeoCoronal Imager, consists of two co-aligned broadband photometric imagers that support simultaneous, common-volume sensing of ultraviolet emission at 121.6 nm (Lyman-alpha) by exospheric hydrogen atoms. Accurate exospheric parameter retrieval from these images requires accurate knowledge of the instrument's optical responsivity, which enables conversion of measured exospheric signal rates into the scientifically relevant quantity of emission radiance. The Carruthers mission achieves absolute sensitivity calibration through the acquisition of photometric measurements of stars and subsequent inversion of the observed fluxes to retrieve the wavelength-dependent responsivity across the passband for each imaging configuration. The retrieval algorithm performance is enhanced by its incorporation of an objective, algorithm-driven ranking criterion to systematically select target calibration stars from a stellar spectral library. The end-to-end workflow, from the stellar selection criterion to the passband inversion, is validated using synthetically generated stellar measurements. These validation tests establish that our optical responsivity retrieval approach has high recovery fidelity, achieving error rates of <5% at the Lyman-alpha wavelength for all primary science imaging modes.