Slit homogenizer introduced performance gain analysis based onSentinel-5/UVNS spectrometer

Abstract. The spectral accuracy of high resolution Earth observation spectrometer missions is affected by the impact of spatially heterogeneous Earth radiance scenes on the instrument spectral response function (ISRF). As the ISRF is the direct link between the forward radiative transfer model and the spectra measured by the instrument, distortions of the iSRF owing to radiometric inhomogeneity of the imaged Earth scene will degrade the precision of the Level-2 retrievals. Therefore, the spectral requirements of an instrument are often parametrized in the knowledge of the ISRF over non-uniform scenes in terms of shape, centroid position of the spectral channel and the Full Width at Half Maximum (FWHM). The Sentinel-5/UVNS instrument is the first push-broom spectrometer that makes use of a concept referred as slit homogenizer (SH) for the mitigation of spatially non-uniform scenes. This is done by employing a spectrometer slit formed by two parallel mirrors, scrambling the scene in along track direction (ALT) and hence averaging the scene contrast only in the spectral direction. The flat mirrors do not affect imaging in the across track direction (ACT) and thus preserve the spatial information in that direction. The multiple reflections inside the SH act as coherent virtual light sources and the resulting interference pattern at the SH exit plane can be described by simulations using scalar diffraction theory. By homogenizing the slit illumination, the SH moreover strongly modifies the spectrograph pupil as a function of the input scene. In this work we investigate the impact and strength of spectrograph pupil variations for different scene cases and quantify the impact on the ISRF stability for different type of aberrations present in the spectrograph optics.