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The Metrology Light Source as a standard for solar radiation

Calibration of the solar spectrometer SPICE of ESA's Solar Orbiter mission using calculable synchrotron radiation

PTB-News 1.2015
08.04.2015
Especially interesting for
  • solar and atmospheric research
  • developers of space telescopes

The SPICE (SPectral Imaging of Coronal Environment) solar spectrometer is part of the Solar Orbiter mission of the European Space Agency whose start is planned for 2017. Within the scope of a research cooperation project which has been concluded with the Max Planck Institute for Solar System Research (MPS) and in cooperation with the Rutherford Appleton Laboratory (RAL) in England, the SPICE engineering model (EM) has recently been calibrated in such a way that it is traceable to the primary source standard Metrology Light Source (MLS). The actual flying model will be calibrated in the summer of 2015.

The SPICE spectrograph, mounted into the large vacuum tank at the MLS

In the past years, PTB has calibrated a series of telescope systems for solar observation and atmospheric research using synchrotron radiation in the UV and the vacuum UV (VUV) spectral ranges for various space missions. Hereby, either individual components or the complete systems were characterized by means of transfer radiation sources at the laboratories of the respective cooperation partner. These transfer radiation sources had been developed especially for this purpose and characterized by means of synchrotron radiation. Now that the entire device has been calibrated directly with the calculable synchrotron radiation from the electron storage ring MLS in a new, large vacuum tank, the workaround via VUV transfer sources has become unnecessary. The calibration chain is thus strongly simplified and contributes to attaining lower uncertainties in the range of a few percent. Since synchrotron radiation exhibits a continuous spectrum, all wavelengths in the spectral range of interest are accessible, contrary to the VUV transfer sources previously used which were based on a hollow cathode discharge and which only emit individual lines.

SPICE allows spatially resolved measurement of the VUV emission of the solar surface and the surrounding corona in the spectral range from approx. 48 nm to 105 nm and serves, among other things, to scientifically investigate the processes of creation and localization of source regions of the solar wind and of the solar magnetic field. For this purpose, the incident solar radiation is imaged onto a narrow and long entrance slit by means of a special VUV multilayer mirror and then dispersed onto two detector arrays through a grating. Spectral information is thus gathered on each array in the direction of dispersion and local information is collected in the orthogonal direction in a narrow band of the solar surface or of the solar corona. By tilting the mirror, entire solar regions can be detected.

Calibration at the MLS was performed at a reduced electron energy in order to suppress short-wavelength false light. Use was also made of the possibility of adapting the spectral radiant intensity to the limit of detection of the sensors via the electron current stored. The linearity of the sensors could thus be investigated, which is of great importance due to the strongly varying line emissions of the sun.

The SPICE Project involves the ESA Member States Germany, France, Great Britain, Norway and Switzerland; besides by ESA, it is also funded by the German Zentrum für Luftund Raumfahrt (DLR – German Aerospace Center), the Centre National d'Études Spatiales (CNES – the French Space Agency), the UK Space Agency, the Norwegian Space Centre, and the Swiss Space Office. The measurements on the SPICE-EM model mainly served to test the calibration scheme and the handling and operation of this complex device together with the corresponding supply infrastructure at the MLS in view of the calibration of the SPICE flying model which is planned for the summer of 2015.

Contact

Roman Klein
Department 7.1 Radiometry with Synchrotron Radiation
Phone: +49 (0)30 3481-7140
E-mail: roman.m.klein(at)ptb.de

Scientific publication

A. Fludra et al.: SPICE EUV spectrometer for the Solar Orbiter mission. Proc. SPIE 8862, 88620F (2013)