CRISTA (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere) is a limb sounding satellite experiment which was flown twice as part of the space shuttle missions STS-66 (November 1994) and STS-85 (August 1997). Infrared emissions of a number of trace gases were measured at wavelengths of 4-71 micrometer in the middle and upper atmosphere in an altitude interval of 10-180 km. The main goal of the two missions was to achieve an unprecedented spatial resolution using three viewing directions.
To allow high measurement speed the instrument is cooled by cryogenic He. During the first flight 23 Si:Ga and three Ge:Ga bulk infrared detectors were operated at temperatures between 2.5 and 13 K. During the second flight 17 Si:Ga, three Ge:Ga bulk and 9 Si:As BIB detectors were used.
Mainly due to the contacts of the detector crystals the measured detector signals show nonstationary behaviour (relaxation effects) which degrade the measured spectra to some extent. A model based on step responses measured during the calibrations of the instrument has been developed for the Si:Ga detectors to reduce these effects. This model has been tested using special measurement modes during the flights as well as calibration spectra.
For the Ge:Ga detectors a method for estimation and correction of occuring relaxation effects is also presented.
Finally the impact of the correction on CRISTA I temperatures and trace gas mixing ratios is shown.