Astronomy

Sandra Adams

ROSAT: Methuselah among satellites

Germany’s ROSAT X-ray satellite began operation on June 1, 1990. Its mirror system, built by Carl Zeiss, was only intended to operate for two to three years. Yet up to a few months ago it was still transmitting sensational pictures back to its ground station.

The optical system, which is used to discover the X-rays, is tubular in shape. Eight mirror shells, made from “ZERODUR®” glass-ceramic are combined to form a nested Wolter telescope. The outside surfaces of the mirrors were etched enabling them to withstand the mechanical stress at blast-off. Even before it was taken into service, the ROSAT mirror system was listed in the Guinness Book of Records as the world’s smoothest mirror. This refers to both the surface of the mirror and also the way the mirror shells are aligned.
SCHOTT supplied eight cylindrical “ZERODUR®” mirror blanks in the 1980s for the mirror system of the successful ROSAT X-ray satellite.
ROSAT has already made many new discoveries. “ROSAT delivered its most valuable pictures at the time when it should really have stopped working,” says Professor Joachim Trümper of the Max Planck Institute for Extraterrestrial Physics in Garching. It has transmitted back to its ground station a map of the sky that never existed before.

ROSAT has discovered more than 100,000 X-ray sources that were previously unknown. Due to ROSAT it has been possible to identify traces of titanium 44 in an explosion cloud from a supernova – an element that has a very short lifespan by cosmic standards. Another spectacular result was the proof of X-ray emissions from “class L brown dwarfs” that were only discovered a few years ago.

ROSAT stopped operation a few months ago. Right up to the last, the optics delivered perfect and brilliant pictures. The satellite was replaced by the Chandra observatory that is also equipped with “ZERODUR®” mirror shells. The highly interesting results from ROSAT, however, will take some years to evaluate fully.