Ultimate sensitivity with ISOCAM

We discuss the instrumental factors which constrain the sensitivity limits of the midinfrared camera (ISOCAM) of ESA's Infrared Space Observatory (ISO ). The observing strategy judged to be best suited to faint-source detectio n is described. A data analysis technique adapted to the extraction of the faintest point-sources is discussed. We report the application of these tec hniques to an extremely deep mid-IR observation of high redshift gravitatio nally lensed objects, in two filters, looking through a lensing galaxy clus ter.