Radiometric Characterization of Space Instrumentation

The Earth’s atmosphere absorbs the short-wave fraction of the electromagnetic spectrum, starting with the ultraviolet (UV) spectral range up to the range of X-rays. This may be an important protection mechanism for life on the surface of the Earth, but it makes the exploration of space more difficult: measurements of the emission of stellar objects in this spectral range can only be carried out outside the Earth’s atmosphere. Hereby, this fraction of the spectrum is of primary importance, not only for the observation and understanding of the astrophysical and solar processes, but also due to the intensive interaction with the Earth’s outer atmosphere and the corresponding climatological and chemical significance of the atmosphere. To ensure reliable and comparable measurements of space-based instruments, these have to be carefully characterized and traced to radiometric standards. For more than 20 years now, PTB has, within the scope of numerous joint research projects with external partners, established itself as the world leader in characterizing space instrumentation by means of synchrotron radiation in the spectral ranges from UV radiation to X-rays. Various technologies were used for the radiometric support of a series of missions – most of them dedicated to solar radiometry. Another important point is, especially in the X-ray range, the characterization of imaging X-ray-optical elements; for this task, the European Space Agency (ESA) ordered its own beamline section. Figure 1 shows a selection of instruments that were characterized or calibrated by PTB using synchrotron radiation. The different colors serve to differentiate instruments where the method is source-based (yellow) [1] from those where it is detector-based (blue) [2]. Starting with the supply and calibration of transfer standards for the Solar and Heliospheric Observatory (SOHO) in 1994 [3], methods and Alexander Gottwald*, Roman Klein, Michael Krumrey, Peter Müller, Wolfgang Paustian, Thomas Reichel, Frank Scholze, Reiner Thornagel Radiometric Characterization of Space Instrumentation