HOLLOW-CATHODE TRANSFER STANDARDS FOR THE RADIOMETRIC CALIBRATION OF VUV TELESCOPES OF THE SOLAR AND HELIOSPHERIC OBSERVATORY (SOHO)

On board the ESA/NASA space mission SOHO (Solar and Heliospheric Obser vatory) there will be several VUV telescope/spectrometer systems to ob serve the solar disk and its corona in the wavelength range 15 nm to 1 60 nm. To determine the absolute spectral sensitivity of these instrum ents prior to launching, transfer source standards have been developed . These transfer standards consist of a high-current hollow cathode so urce combined with collimating optics. One standard uses a concave mir ror at normal incidence for the wavelength range 50 nm to 160 nm and t he other standard uses Wolter type II grazing incidence optics for the range 16 nm to 80 nm. These transfer source standards are calibrated against the electron storage ring BESSY as a primary radiometric sourc e standard.