The ESA Sentinel-4 Mission is part of the European Commission Copernicus Programme for monitoring the Earth. It is going to provide free data on trace gas concentrations and aerosols in the atmosphere to support real time operational services of air-quality for Europe. The Sentinel-4/UVN Instrument will be placed in a geo-stationary orbit on board of the Meteosat Third Generation Sounder (MTG-S) satellite. Jena-Optronik has been contracted to develop and build the core main optics of the Sentinel-4/Ultra-violet/Visible/Near- Infrared (UVN) instrument, which comprises a telescope, a UV-VIS spectrometer and a NIR spectrometer. Due to its geo-stationary orbit and its compactness the photon flux seen by the instrument is low. Simultaneously the lens based optics of the instrument set new standards concerning performance. In consequence straylight levels within the instrument need to be reduced to the bare minimum including high quality polishing of the optics as well as manufacturing facilities as well as processes, which reduce the overall allowed contamination of the optical surfaces to be <50ppm for particular and <50ng/cm² molecular contamination. Furthermore, the very demanding performance requirements led to an instrument design, which sets the demands on manufacturing and alignment quality to a new level. To realize the instruments performance over the complete specified thermal range Jena-Optronik developed a unique passive thermal focus compensation for the lens optics, which acts like a passive auto-focus system w.r.t. thermal changes. Moreover, we used precision lens mount manufacturing techniques to achieve the required lens positioning accuracy of less than 5μm. The design of the instrument will be presented as well as the highlights from the assembly, integration and test campaign of the three core optics. All three PFMs, the telescope as well as the UV-VIS and the NIR spectrometers have been successfully delivered to Airbus Defence & Space after ending their environmental test campaign successfully.

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Simon Chelkowski, Steffen Kirschstein, Torsten Hertel, Ute Gawronski, Steffen Rienecker, and Eyk Gebhardt, "SENTINEL-4 combined optical assemblies," Proc. SPIE 11852, International Conference on Space Optics — ICSO 2020, 1185233 (Presented at International Conference on Space Optics — ICSO 2021: 11 June 2021; Published: 11 June 2021); https://doi.org/10.1117/12.2599530.