03 Sep. Another ESA mission launch with optics from Jena
Just a few months after the launch of Sentinel-4, Sentinel-5 is now the next mission in the Copernicus series to be launched into space. The Sentinel-5 instrument is not a standalone satellite, but rather a UVNS (Ultraviolet Visible Near-infrared Shortwave) spectrometer that was launched into low Earth orbit on a Metop satellite from Eumetsat. From there, Sentinel-5 will monitor global air quality, trace gases, and aerosols in the Earth’s atmosphere on a daily basis.
Sentinel-5 analyzes the light reflected from Earth in four spectral channels: ultraviolet, visible, near-infrared, and shortwave infrared. The Fraunhofer Institute for Applied Optics and Precision Engineering IOF contributed several optical components for the near-infrared channel (685 to 710 nm): a spectrometer grating, space-qualified mounts, and an adjusted lens group.
Optical mounts for extreme conditions
Via an entrance telescope light is collected and directed into the interior of the instrument via a collimating lens and deflection mirror. These optics are held in place by isostatic mounts made of titanium, which were developed and manufactured at Fraunhofer IOF. Andreas Kamm, Fraunhofer researcher in optical and mechanical system design, explains: “The mount must not transfer mechanical stresses, which occur due to integration and operating temperature in space, to the optics, as this would lead to deformation and thus impair their imaging quality.” To avoid this, the researchers developed a two-stage adjustment and bonding process as well as special solid-state joint arrangements that mechanically decouple the optics from the mount and at the same time ensure that they survive the extreme conditions of rocket launch without damage.
One of the two deflection mirrors, glued into a titanium holder. The solid-state joints can be seen on the right and left in a V-shape. Image: Fraunhofer IOF
Very efficient light diffraction
Inside, the light hits an assembly consisting of a prism and an optical grating. This grating was manufactured at Fraunhofer IOF. “For Sentinel-5, we manufactured a transmission grating with a particularly high diffraction efficiency of more than 95% and less than 2% polarization sensitivity,” explains Dr. Falk Eilenberger, head of the Micro- and Nanostructured Optics Department. “The grating is made of quartz glass, into which precise grooves with a width of 197 nm were incorporated using electron beam lithography and reactive ion etching.”
Using a special mounting system, technically similar to that used for the collimating lens and the deflection mirrors, the optical grating is connected to a prism located below it. The mechanical requirements for this assembly were even higher, explains Kamm: “The grating had to be adjusted with high precision in five degrees of freedom, with a wedge-shaped air gap to the prism, and this state had to be fixed in a stable and tension-free manner.”
Optical grating for the near-infrared channel of the Sentinel-5 spectrometer. Image: Fraunhofer IOF
Lens group precision-turned to size
After the prism-grating assembly, the diffracted light finally hits a lens group, which transmits it to the spectrometer’s camera. The lens mounts with lenses already glued in place were reworked using an alignement turning machine developed at Fraunhofer IOF. This special process allowed the distances and alignments of the lenses to each other to be adjusted with an accuracy of a few micrometers.
A Stack of adjusted lenses during integration. Image: Fraunhofer IOF
Fraunhofer IOF: Part of the Sentinel family
Fraunhofer IOF has been involved in various Sentinel missions for many years: the institute supplied optical filters for Sentinel-2 and a reflection grating for Sentinel-4. It also contributed three optical assemblies per satellite for the spectrometer of CO2M (Sentinel-7), a supplementary mission in the Copernicus program.
Fraunhofer IOF’s work on the components was carried out in close and long-standing cooperation with Jena-Optronik.
Fully assembled module consisting of the grating (on top) and a prism underneath (not visible here). A special mount (silver frame + V-shaped joints) connects the grating and prism. Image: Fraunhofer IOF
Source: www.iof.fraunhofer.de
Image: ESA/ATG medialab/Fraunhofer IOF