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Lens clamping is related to the object and image plane of the optical assembly, thus high stability of the lens positions within the lens tubes are required to maintain stable image plane. Axial shift of each lens has to be less than 3 um. Soft clamping mechanism was used that consists of a spring ring. This allows a good control of the installed preload and also reduced the CTE effects. The spring ring consists of six uniformly distributed elements that are made out of stainless steel allowing clamping force up to 10 kN. Titanium pressure ring in the spring ring reduces surface pressure from subsequent aluminum parts and also allows to avoid corrosion effects.
Stray Light
Stray light is a factor in any optical instrument. In the case of Sentinel-4 UVN, the main stray light is present in the UV region. Light in the instrument passes 37 surfaces, which causes a concern for introducing stray light within the instrument. Stray light analysis was performed to obtain optimal design. In addition to that also extensive surface polishing on the refractive elements were performed to obtain surface roughness that was almost less than 0.5 nm RMS. Black coating was applied to all stray-light relevant parts, to absorb the stray-light.
The Sentinel-4 mission demands a very precise and high performance optical system, which involves many challenges that require creative and reliable solutions. Jena-Optronik company was responsible for designing, ensuring manufacturing compliance and testing of the main optical assembly of the Sentinel-4 mission. They have described their results in an article upon which the presentation was based on. They were able to deal with different complex problems, simulate the results and then achieve a final assembly that was tested and was able to perform in the required operational margins.
Papers:
Link to presentation:
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