Improving stray-light characterization and correction beyond blooming, the experience of the fluorescence imaging spectrometer optical model refurbished

The fluorescence explorer European Space Agency mission with its payload, the fluorescence imaging spectrometer (FLORIS), aims to perform quantitative measurements of the solar induced chlorophyll fluorescence from space, with the purpose of improving the monitoring of the health of Earth vegetation. The retrieval of a faint signal, such as the one from chlorophyll fluorescence, requires very low stray-light (SL) levels. The SL reduction in FLORIS implied constraints impacting the design, by means of using low roughness optical components; the integration, through a strict contamination control; and the data processing with the need of a very accurate correction strategy making use of numerical models well correlated with experimental data, to be acquired during the on ground calibration activities. To assess and validate the correction strategy, an accurate SL characterization has been anticipated during the FLORIS optical model refurbished campaign. Different methods, such as out-of-field and out-of-band measurements, have been investigated to avoid the detector blooming affecting measurements with high input signals. By combining the results from the different approaches, it has been possible to achieve up to 9-10 orders of magnitude of explored dynamic range. The model, correlated with measurements, has finally proved to be capable of correcting SL with a reduction factor of the order of 10, down to a level of the order of 30% of the required fluorescence error (10% of a fluorescence level of about 2 mW m−2 sr−1 nm−1).