Solar radiometry with the telescope and vacuum-ultraviolet spectrometer SUMER on the Solar and Heliospheric Observatory (SOHO)

SUMER, the Solar Ultraviolet Measurements of Emitted Radiation instrument, is part of the payload of the Solar and Heliospheric Observatory (SOHO) whi ch began its scientific mission in January 1996. SUMER takes spatially and spectrally highly-resolved images of the Sun in the spectral range from 46 nm to 161 nm. It investigates structures and associated dynamic processes o f the solar atmosphere from the chromosphere through the transition region to the inner corona, over a temperature range from 1 x 10(4) K to 2 x 10(6) K. Absolute radiance measurements, which are used to derive plasma tempera tures and densities in the solar atmosphere, require that the spectral resp onsivity of the instrument be known. We describe the radiometric pre-launch calibration of SUMER, which is traceable to the electron storage ring BESS Y, and the measurements performed to confirm and extend the laboratory cali bration during the first year of operation. These measurements put SUMER in a position to measure at certain wavelengths solar vacuum-ultraviolet spec tral radiance, and also irradiance, with a relative uncertainty of 0.15 (1 sigma).