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 Observatory) there will be several VUV telescope/spectrometer systems to observe the solar disk and its corona in the wavelength range 15 nm to 160 nm. To determine the absolute spectral sensitivity of these instruments prior to launching, transfer source standards have been developed. These transfer standards consist of a high-current hollow cathode source combined with collimating optics. One standard uses a concave mirror at normal incidence for the wavelength range 50 nm to 160 nm and the 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 source standard.