Week 10/2021

Detailed view from space

Jena-Optronik is contributing several optical systems to Sentinel-4 that allow the taking of high-resolution images of our planet Earth. © Florian Brill for Jena-Optronik GmbH

Detecting things on Earth as small as 8 kilometers from orbit at an altitude of 36,000 kilometers? This is what the latest European Meteosat Third Generation Sounder satellite (Meteosat Third Generation, MTG for short) is capable of while on the European Copernicus program’s Sentinel-4 mission. With eyesight like that, one could decipher the last line of letters on a typical eye test poster from across a football stadium. Jena-Optronik GmbH, a company from Thuringia, builds the optics and electronics involved. To make Sentinel-4 possible, the Jena experts have developed two innovative procedures.

Jena-Optronik is contributing several optical systems to Sentinel-4, including a telescope and two spectrometers that capture high-resolution images of our planet Earth. These optics work in principle like their terrestrial counterparts: in an objective, different lenses are arranged one behind the other to focus the light, ultimately creating sharp images. However, the conditions under which the lenses operate in space are much more demanding than on Earth. Jena-Optronik has developed two technologies to handle the Sentinel-4 task that are unique worldwide.

Stable despite increased mechanical loads

One is called clamping, a process for assembling the objectives. The challenge? On rockets, especially during launch, the objective is exposed to extremely high loads. These usually cause the lens holders inside the objective housing to bend or lose their alignment to each other. This causes the optics to fall out of focus and the whole instrument to become unusable. Jena-Optronik has therefore developed a special clamping system and mechanical holders for the lenses. During the assembly of the lenses into the tube, a pressure of one ton is generated. This ultimately leads to the final adjustment and securing of the correct spacing of the lenses. All the lenses are fixed by pressure, so to speak, and thus prepared for the high accelerations and loads during rocket launch.

Sentinel-4 will join ESA's Copernicus mission from 2023. © ESA – P. Carril

Sharp images despite temperature fluctuations

For an objective to be able to fulfill its task in space, Jena-Optronik has also developed a process that goes by the unwieldy name of passive athermalization. Background: A geostationary satellite is exposed to regular temperature fluctuations, changing according to whether it is flying in the Earth's shadow or is exposed to full sunlight. This causes its materials to expand and contract. In an objective, this would change the distances between the lenses and thus the focus continuously, which must not happen. Even deviations in the µm range, around 0.001 millimeters, would lead to blurred images. In order to prevent this, so far the distances between lenses have been actively readjusted, for example using software and motor-driven actuators. This process is as complex as it is error-prone, requiring additional electronics and sensors. Jena-Optronik has now developed a lens mount in which each lens has a two-part mount ring that automatically counteracts any deformation resulting from temperature changes. In this way, temperature fluctuations can be compensated for without the need for a power source or an additional drive mechanism.

Jena-Optronik has developed both technologies on behalf of Airbus Defence & Space and the European Space Agency ESA. They are expected to be deployed on the latest MTG satellite in the Sentinel-4 mission starting in 2023. This will then monitor the Earth's atmosphere as part of the European Copernicus Earth observation program, collecting valuable data on greenhouse gases, air quality and ozone formation, among other things. This will further improve the already highly accurate predictions of climate change and enable comprehensive protective measures to be taken.

This article has been produced with the financial assistance of the European Union. The views expressed herein can in no way be taken to reflect the official opinion of the European Union or ESA.