The in-flight monitoring and validation of the SOHO CDS Normal Incidence Spectrometer radiometric calibration
The scientific return from an extreme-ultraviolet spectrometer depends on the accuracy and precision of its radiometric calibration. For the Coronal Diagnostic Spectrometer on SOHO, radiometric calibration started pre-launch in the laboratory and continued after launch by making comparison measurements of the same area of the Sun with calibrated sounding rocket payloads and also by intercalibration with the SUMER instrument on SOHO. The present work uses the measurement of line ratios to monitor and validate the calibration over the first six years of observation. As well as using branching ratios and line ratios independent of the electron temperature and density, line ratios dependent on electron temperature or density have also been used successfully to validate and monitor the calibration. The results indicate that, within the uncertainties, the radiometric calibration has been validated and maintained over the first six years of observations apart from three specific wavelengths, 338.98 Å, 315.0 Å, and 311.8 Å. Problems with lines at 608.4 Å, 303.4 Å (seen in second order), 335.4 Å, and 360.7 Å are attributed to difficulties with the burn-in correction.