Modelling Transient Effects in the IR Array of the Short Wavelength Channel of ISOCAM

Abstract

The short-wave channel (SW) of ISOCAM, the IR camera on board the ISO satellite, is equipped with a 32 × 32 CID InSb array. This array shares with other detectors of ISO instruments the characteristic of being subject to some time lag when responding to a flux variation. Known as transient effect, this default which is not quite understood (in particular, it does not correspond to a simple time constant), must be corrected for in data processing. Starting from the remark that the lower the signal, the longer the equivalent time constant, we have developed a mathematical model that reproduces quite well the observed behaviour, using only a small set of parameters. In practice, the model consists of solving a functional equation. The model provides the asymptotic value of the stabilized signal, which is in general the information one looks for. An a posteriori analysis of the form of the functional equation suggests that the effect relates to surface traps in the semi-conductor, as a rough estimation would confirm.