The European Space Agency (ESA) SENTINEL-4 mission with main goal to measure gas concentration and aerosols in the atmosphere over Europe that will allow to monitor real-time air quality. The core technology is the Sentinel-4 ultra Ultraviolet-Visible-Near-Infrared (UVN) light imaging spectrometer instrument. The Sentinel-4 UVN will be integrated in the Meteosat Third Generation Sounder (MTG-S) satellite that will be placed in the geostationary orbit over Europe. The main optics assembly are built by Jena-Optronik company, which include a telescope, UV-VIS spectrometer and a near-infrared (NIR) spectrometer. The Sentinel-4 instrument itself is built by Airbus company. The focus of the presentation was the overall description of the optics assembly and the main challenges related to the design.
The optical assembly consists of a high performance telescope that also separates the light in two spectral bands, two spectrometers and two focal plane assemblies (FPAs). The two spectral bands are 305-500 nm and 750-775 nm, which are then measured by the UV-VIS and NIR spectrometers, respectively. There were four main challenges:
Passive Athermalization
The ultraviolet optics for the telescope assembly requires CaF2 and SIO2 doublet lenses for proper chromatic correction. However, these materials have different coefficients of thermal expansion (CTE). The required operational temperature for the design was 17 to 23 degrees Celsius, but the initial design lead to operational temperature range from 19.8 to -20.3 degrees Celsius due to the difference in CTE. This result was unacceptable.
A mechanism to compensate the thermal shifts was introduced to achieve the required operational temperature range. The mechanism involved passive actuation involving complex lever mechanism that was dependent on the temperature. This allowed shifting the inter-lens distance depending on the temperature.
Lens Gluing
The optical performance of the mission demands almost nominal optical performance. Only very small deformation in the lenses are allowed. Due to these reasons the manufacturing process for all optical elements are optimized, and the mounting of the lenses also should not deteriorate the optical performance. To address these concerns, a glue having practically the same CTE as the refractive material was used to avoid having temperature induced stresses after mounting that may affect the performance. Simulated stress analysis allowed to use optimized gluing points for the lenses. Afterwards, long term tests were performed to verify the performance of the proposed mounting mechanism and the optical performance.
Lens Centering
As mentioned previously, the mission demands very precise and efficient optical performance. Thus, high accuracy positioning and centering for each optical element is required. The distortion maximum for multiple elements go as low as 1.4 um to 2.1 um. The main concern regarding this was the aluminum barrel and the centering material because this materials produced a risk of sticking during the joining process. To avoid the sticking, Ni plating was used for the metals to obtain metal mirror quality.
Lens Clamping
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.
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Link to presentation: