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).
KEYWORDS: Equipment, Charge-coupled devices, Fluorescence, Signal detection, Astronomical imaging, Frequency modulation, Fermium, Design and modelling, Light sources and illumination, Data modeling
The FLuorescence EXplorer (FLEX) European Space Agency (ESA) mission with its payload, the FLuORescence Imaging Spectrometer (FLORIS), aims at performing 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, but also 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 (OGC) activities. In order to assess and validate the correction strategy, an accurate SL characterization has been anticipated during the FLORIS Optical Model Refurbished (OMR) campaign. Different methods, such as out-of-field and out-of-band measurements, have been investigated in order 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 decades of explored dynamic range. The model, correlated with measurements, has finally proved to be capable to correct SL with a reduction factor <10, down to a level less than 40% of the required fluorescence error (10% of a fluorescence level of about 2 mW/m2/sr/nm). Additional presentation content can be accessed on the supplemental content page.
The exposure of Charge-Coupled Devices (CCD) to high-energy particles in space leads to a degradation of their performances. One of the observed mechanisms is the creation of defects in the CCD silicon lattice by displacement damage, inducing a reduction of the Charge Transfer Efficiency (CTE), i.e. the ability of the device to efficiently transfer the photo-induced charge to the read-out output node. Hence a reduction of the imaging quality of the detector. We present here a comparison of the modelled and measured optical quality of the FLEX CCD exposed to a high energy proton flux. The optical quality was directly measured on an irradiated flight representative device. A physical model of the detector, including an accurate modelling of the charge trapping dynamic, is used to generate synthetic scenes affected by CTE degradation from which the optical quality is assessed and compared to the measurement. Eventually the correlation of the model and the measurement will allow to accurately assess the performances of a detector exposed to space radiation environment.
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