The solar package on ISS: SOL-ACES

The "solar package" comprises the experiments SOLSPEC (UV/Vis to IR spectra l range), SOVIM (total solar radiation) and SOL-ACES to be installed on a C oarse Pointing Device (CPD) of the international Space Station for a 1.5 ye ars mission launched in 2002. The CPD allows for measuring periods of at le ast 15 minutes per orbit totaling approximately 600 hours per year of solar observations. The Solar Auto Calibrating EUV/UV Spectrometers (SOL-ACES) a re currently developed to measure the solar radiation (full disk) in the 17 nm to 220 nm spectral range with four grazing incidence grating spectromet ers. To obtain high radiometric accuracy of better than 10 %, a double ioni zation chamber is assigned to each of the spectrometers as primary detector standard. Optical bandpass filters are mounted on a filter wheel to be pla ced at the Entrance apertures of the spectrometers and ionization chambers and thereby will establish the radiometric link between these devices. The spectrometers are designed as scanning monochromators operating at fixed in cidence angles. The deflected radiation is monitored by rotating an assembl y containing a parabolic mirror, an exit slit and a channel electron multip lier around the grating center. The optical length of the ionization chambe r of 0.5 m is divided into two identical electrode sections. In addition, t he transmitted radiation is measured by a silicon diode detector located at the end of the absorption path. Detector and electrode signals are recorde d as a function of the gas pressure in the chamber, which is increased from zero to a few hectopascals during a single measurement. This data permits the absolute quantification of the radiant solar flux in the wavelength int erval transmitted by the bandpass filter and the correction for secondary e ffects, such as ionization caused by photoelectrons. With these measurement s the spectrometer efficiencies at the filter bandpass wavelengths can be r ecalibrated as required during the mission. (C) 2000 Elsevier Science Ltd. All rights reserved.