Radiometric Calibration Tracking of the Vacuum-Ultraviolet Spectrometer SUMER During the First Year of the SOHO Mission.

Detailed radiometric calibration tracking of the vacuum-ultraviolet spectrometer SUMER (from solar ultraviolet measurements of emitted radiation) was performed during the first year of the Solar and Heliospheric Observatory (SOHO) mission and will continue. In view of the flight history of many previous solar UV instruments, the stability of calibration of the extreme-ultraviolet instruments on SOHO has been a major concern. Results obtained during the first year of operation show that excellent radiometric stability has been achieved with SUMER. These results were accomplished by stringent cleanliness and contamination-control procedures during all phases of the project. We describe the strategy and results of the in-flight calibration tracking program performed with SUMER.