MODELLING TRANSIENT EFFECTS IN THE IR ARRAY OF THE SHORT WAVELENGTH CHANNEL OF ISOCAM

The short-wave channel (SW) of ISOCAM, the IR camera on boardthe ISO satellite, is equipped with a 32 × 32 CID InSb array. This arrayshares with other detectors of ISO instruments the characteristic of beingsubject to some time lag when responding to a flux variation. Known astransient effect, this default which is not quite understood (in particular, itdoes not correspond to a simple time constant), must be corrected for in dataprocessing. Starting from the remark that the lower the signal, the longer theequivalent time constant, we have developed a mathematical model thatreproduces quite well the observed behaviour, using only a small set ofparameters. In practice, the model consists of solving a functional equation.The model provides the asymptotic value of the stabilized signal, which is ingeneral the information one looks for. An a posteriori analysis of theform of the functional equation suggests that the effect relates to surfacetraps in the semi-conductor, as a rough estimation would confirm.