SOL–ACES: Auto-calibrating EUV/UV spectrometers for measurements onboard the International Space Station

Abstract The SOL–ACES experiment is prepared to be flown with the ESA SOLAR payload to the International Space Station as planned for the Shuttle mission E1 (Columbus) in August 2006. Four grazing incidence spectrometers of planar geometry cover the wavelength range from 16 to 220 nm with a spectral resolution from 0.5 to 2.3 nm. These high-efficiency spectrometers will be re-calibrated by two three-signal ionization chambers to be operated with 42 band pass filters on routine during the mission. Re-measuring the filter transmissions with the spectrometers also allows a very accurate determination of the changing second order (optical) efficiencies of the spectrometers as well as the stray light contributions to the spectral recording in different wavelength ranges. In this context the primary requirements for measurements of low radiometric uncertainty are discussed in detail. The absorbing gases in the ionization chambers are neon, xenon and a mixture of 10% nitric oxide and 90% xenon. The laboratory measurements confirm very high count rates such that optical attenuators have to be applied. In addition, possible interfering contributions to the recorded data as generated by secondary effects can be determined to a high degree of accuracy by this method. Hence, very accurate irradiance measurements are expected in terms of relative standard uncertainties (RSU) ranging from 5% to 3% depending on the wavelength range.