1999A&A...342..313A

We have developed a new ISOCAM data reduction technique based on wavelet analysis, especially designed for the detection of faint sources in mid-infrared surveys. This method, the Pattern REcognition Technique for Isocam data (PRETI) has been used to reduce the observations of the Hubble Deep Field (HDF) and flanking fields with ISOCAM at 6.75 (LW2) and 15µm (LW3) (Rowan-Robinson et al., 1997MNRAS.289..490R). Simulations of ISOCAM data allow us to test the photometric accuracy and completeness of the reduction. According to these simulations, the PRETI source list is 95% complete in the 15µm band at 200µJy and in the 6.75µm band at 65µJy, using detection thresholds which minimize the number of false detections. We detect 49 objects in the ISO-HDF at high confidence secure level, 42 in the LW3 filter, 3 in the LW2 filter, and 4 in both filters. An additional, less secure, list of 100 sources is presented, of which 89 are detected at 15µm only, 7 at 6.75µm only and 4 in both filters. All ISO-HDF objects detected in the HDF itself have optical or infrared counterparts, except for one from the additional list. All except one of the radio sources detected in the field by Fomalont et al. (1997ApJ...475L...5F) are detected with ISOCAM. Using a precise correction for the field of view distortion of ISOCAM allows us to separate blended sources. This, together with the fact that PRETI allows to correct data on the tail of cosmic rays glitches, lead us to produce deeper source lists than previous authors. Our list of bright sources agree with those of Desert et al. (1999A&A...342..363D) in both filters, and with those of Goldschmidt et al. (1997MNRAS.289..465G) in the LW3 filter, with systematic difference in photometry. Number counts derived from our results show an excess by a factor of 10 with respect to the prediction of a no evolution model (Franceschini) in the LW3 band. On the contrary, the number of sources in the LW2 band is compatible with the prediction of such a model, but with greater uncertainties, given the small number of detections.