This paper reports on the development of a sun simulator (SUSI) as an optical ground support equipment (OGSE) for the calibration and characterization (CC) of the Sentinel-5 spectrometers. The illumination system is based on a Xenon continuous light source, a homogenizer and a telescope. A Xenon plasma source is significantly non-uniform, both spectrally and spatially. To meet the stringent spatial homogeneity requirements, a large homogenizer based on a kaleidoscope configuration was designed and manufactured. The proper collimation and large exit pupil are achieved through a mirror-based Offner telescope, avoiding large incidence angles, which would affect spatial uniformity and polarization. SUSI covers a very wide spectral range, from 270 to 2300nm, with an irradiance of 80 mW/cm2. SUSI provides an illuminated exit pupil with a very high spatial uniformity. The collimated beam of 0.27º divergence illuminates an exit pupil 160mm diameter with continuous light, reaching a non-uniformity of irradiance better than 1% for most spectrum (according to IEC 60904-9 definition) which is, to the best of the authors’ knowledge, the highest level of spatial uniformity for large aperture sun simulators. A breadboard was manufactured to test and validate the performance of the homogenizer coupled with the Xenon light. Due to the large diameter of the exit pupil, a set of tests were specifically developed to evaluate the performance across the entire aperture and spectral range. Using a fiber-tip based spectrometer, the exit pupil was scanned to measure the irradiance profile. Results from the breadboard showed a performance better than 1% spatial uniformity at 0.27º divergent beam, for the spectral range 270nm to 800nm. The ray tracing model was compared and validated with the experimental data. Simulation results showed an equivalent performance for SWIR1 and SWIR3 spectral bands. The Critical Design Review (CDR) was successfully achieved, and SUSI is currently being assembled. This work will present the development of the instrument, details on the tests developed to evaluate its performance, and the experimental results of the upcoming test campaign.

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J. Mendes-Lopes, J. Meyer, P. Borges, J. Pereira, R. Henriques, J. Pimentao, and D. Mesquita, "Sentinel-5 OGSE: large aperture sun simulator," Proc. SPIE 11852, International Conference on Space Optics — ICSO 2020, 118524E (Presented at International Conference on Space Optics — ICSO 2021: 11 June 2021; Published: 11 June 2021); https://doi.org/10.1117/12.2599653.