The coronal diagnostic spectrometer for the solar and heliospheric observatory: Experiment description and calibration

The Coronal Diagnostic Spectrometer (CDS) to be flown aboard the ESA/NASA Solar and Heliospheric Observatory (SOHO) is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet (EUV) wavelength range 15 nm to 80 nm. By observing the absolute and relative radiance of selected lines and line profiles, we will be able to derive temperature, density, flow and abundance information for the plasmas in the solar atmosphere. Spatial and temporal resolutions of down to a few arcseconds and 1 second, respectively, allow such studies to be made within the fine-scale structure of the solar corona. Simultaneous coverage of large-wavelength bands provides the capability for simultaneously observing the properties of plasmas across the wide temperature ranges of the solar atmosphere. The pre-launch calibration is achieved through the use of a hollow cathode discharge source which is used as a transfer standard to allow calibration of the CDS against the primary standard of the BESSY electron storage ring in Berlin. The transfer source provides a radiation source of calibrated radiant flux stable to a few percent. By the use of different selected gases in the discharge tube, each of the detector wavelength intervals in the CDS can be covered adequately. The pre-delivery calibration of the CDS has been performed and some results are shown.