SCHOTT glass for X-ray telescope

For astronomers, black holes always pose a challenge. Because they swallow up all of the available light, it is virtually impossible to observe them. However, they do give away their position when ele…

For astronomers, black holes always pose a challenge. Because they swallow up all of the available light, it is virtually impossible to observe them. However, they do give away their position when electromagnetic radiation escapes from their surrounding environment. Efforts are now being made to visualize these more effectively with the help of the new NuSTAR (Nuclear Spectroscopic Telescope Array) X-ray telescope for which SCHOTT has supplied a mirror substrate made of razor-thin specialized glass for optics developed by the National Space Institute (DTU Space) in Copenhagen (Denmark), Columbia University, New York and Goddard Space Flight Center (GSFC), Greenbelt (USA). In the Wolter Telescopes used for NuSTAR (which will be space-based to avoid atmospheric absorption of X-radiation) electromagnetic radiation passes through a set of nested mirror shells. Due to the extremely flat angle of entry, the radiation is not absorbed, but rather reflected, and can be guided onto the detector. This not only produces a two-dimensional image, but also measures the energy contained in the photons as they arrive. This type of spectroscopy allows for conclusions to be made on the interior atomic structure, temperature, as well as chemical and physical processes inside the field of view. Researchers at DTU Space in Copenhagen, Columbia University in New York and GSFC, Greenbelt (USA), have now created the prerequisites for much better images of the depths of the universe, thanks to a new technology. “New mirrors made of thermally formed, thin coated glass are our most important innovation”, said Finn Erland Christensen, Scientific Director of the Astrophysics Department at the DTU. “These can reflect X-radiation with ten times more energy than the telescopes currently being used at the NASA mission Chandra and ESA mission XMM-Newton. This provides us with a 100-fold increase in sensitivity in the hard X-ray band”, he added. For the telescope optics, Mr. Christensen and his team used mirror shells that consist of 0.21 millimeter thin glass from SCHOTT, arranged in multiple layers. “Thin glass is light, flexible and stabile and can be thermally formed into near conical segments. Most importantly, however, it offers a much better surface structure than metal or plastic – the roughness is less than 5 atomic radii. This means we can coat these surfaces directly with the specialized X-ray reflecting multilayer structures which enable reflection in the hard X-ray band”, Mr. Christensen said. “The required tolerances were between 0.4 and a maximum of 0.6 nanometers. By comparison, the smallest structures that can be detected using a conventional light microscope are 200-500 nanometers in size. However, with an astronomical telescope mirror, stripes or waves only nanometers in size can result in too much distortion”, explained Oliver Jackl, product group head at SCHOTT Electronics & Biotech in Grnenplan (Germany). “Due to the continued further development of our drawing technique, we are able to guarantee surface roughness of less than 1 nanometer. This is really what makes new high-tech applications possible to begin with”, he added. SCHOTT “D 263 T” thin glass, a colorless borosilicate glass with high chemical resistance, was used to manufacture the mirror substrate. Thanks to a special manufacturing technique, it can be a mere 0.03 mm to 1.1 mm in thickness. The razor-thin glass is normally used for infrared filters in cameras, as a touch-sensitive panel in navigation instruments inside automobiles or as opto caps for laser diodes and opto-electronic sensors. The new thin glass mirrors are installed on the NASA space probe NuSTAR which is scheduled to orbit the earth and map the distribution of black holes in the universe from 2011. NASA“s NuSTAR mission is managed by the Jet Propulsory Laboratory (JPL). Principal investigator is Prof. Fiona Harrison of the California Institute of Technology (Caltech) in Pasadena (USA). NuSTAR consists of two X-ray telescopes that are parallel aligned. Observation takes place through Wolter Telescopes that are mounted onto an arm that can be extended to a focal length of ten meters. These concentrate the radiation onto two detectors that are found on the body of the satellite. From here, the images are transmitted down to earth. The improved technology of the telescopes continuously enables new discoveries. Whereas slightly more than 300 sources of X-ray radiation could be counted in the sky back in the 1970s, some 500,000 are known today.