SPACE OPTICS
Sturdy optics readied for weather satellites
Carl Zeiss (Oberkochen, Germany) is producing the optics for three weather satellites of the Meteosat 2 generation. The order is worth a total of DM18 million ($10 million) for the company. The satellites will be orbiting Earth from the year 2000 onward, making weather forecasts more precise. The European Space Agency (Paris, France), Matra Marconi Space (Toulouse, France), and Daimler-Ben¥Aerospace (Munich, Germany) are in charge of the project, named the Spinning Enhanced Visible and Infrared Imager.
The three satellites are scheduled to be launched in 2000, 2002, and 2006 to transmit pictures of the earth from a geostationary orbit. The satellites will rotate more than a hundred times a minute while scanning the earth`s surface to document the weather.
High stress, accuracy requirements
An optical system consisting of three imaging mirrors and a plane-scanning mirror along with the mechanical components must be able to cope with the extremely high stress in space and meet the highest accuracy requirements. The microroughness of the mirror surface lies well below 1 nm. Such critical requirements for polishing are met by the glass ceramic material Zerodur, fabricated by Schott Glasswerke (Mainz, Germany). This material also displays outstanding thermal stability. Within a very broad range of temperature, Zerodur does not change its shape and therefore meets the conditions necessary for creating shar¥and flawless pictures.
Both size and weight of the optical system are important to its suitability for space (see photos). The head of the Oberkochen Space Technology Division, Joachim Schulte, explains that Carl Zeiss is treading new paths. The primary mirror has a diameter of 0.5 m. With dimensions measuring 83 ¥ 53 cm, the scanning mirror is a respectable size for a space telescope.
Weight reduction
The weight of the mirror blocks had to be substantially reduced. More than 70% of the material was removed in a milling process, leaving just enough to guarantee the stability of the optical surface. Nevertheless, with a wall thickness of only a few millimeters, the mirror blocks are able to withstand the stress of the launch. Furthermore, the optics must perform perfectly for several years in space.
Achim Strass
Contributing Editor, Germany