Solar spectroradiometry with the telescope and spectrograph SUMER on the Solar and Heliospheric Observatory SOHO

Since the beginning of 1996, the vacuum-ultraviolet telescope and spectrogr aph SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on the Sola r and Heliospheric Observatory (SOHO) of ESA and NASA, has obtained radianc e spectra of many features of the quiescent and active Sun with high spectr al and spatial resolutions. Irradiance measurements of line emissions have also been performed in the wavelength range 58.4 nm to 154.8 nm. The instru ment was calibrated against the electron storage ring BESSY I as a primary source standard with the help of a transfer source standard based on a holl ow-cathode discharge lamp. A thorough cleanliness programme, specifically a imed at chemical contamination control, resulted in an excellent radiometri c stability of the normal-incidence optical system as well as of the detect ors. This was verified under operational conditions by various techniques e mployed during the SOHO mission, such as line-ratio studies, star observati ons and comparisons with other instruments. The observations provide vacuum -ultraviolet radiometry of the Sun in many emission lines of atoms and ions with a relative uncertainty of 0.15 (1 sigma) for the wavelength range 54 nm to 125 nm.