A classical approach to faint extragalactic source extraction from ISOCAM deep surveys - Application to the Hubble Deep Field

We have developed a general data reduction technique for ISOCAM data coming from various deep surveys of faint galaxies. In order to reach the fundame ntal limits of the camera (due to the background photon noise and the reado ut noise), we have devised several steps in the reduction processes that tr ansform the raw data to a sky map which is then used for point source and s ligthly extended source extraction. The main difficulties with ISOCAM data are the long-term glitches and transient effects which can lead to false so urce detections or large photometric inaccuracies. In many instances, redun dancy is the only way towards clear source count statistics. A sky pixel mu st have been "seen" by many different CAM pixels. Our method is based on le ast-squares fits to temporal data streams in order to remove the various in strumental effects. Projection onto the sky of the result of a "triple-beam method" (ON -(OFF1 + OFF2)/2) obtained from the signal of a given pixel du ring three consecutive raster positions leads to the removal of the low fre quency noise. This is the classical approach when dealing with faint source s on top of a high background. We show illustrative examples of our present scheme by using data taken from the publicly available Hubble Deep Field I SOCAM survey in order to demonstrate its characteristics. More than thirty sources down to the 60 (resp. 100) mu Jy level are clearly detected above 4 sigma at wavelengths of 7 (resp. 15) mu m, the vast major ity at 15 mu m. A large fraction of these sources can be identified with vi sible objects of median magnitude 22 and K-band magnitude of 17.5 and redsh ifts between 0.5 and 1 (when available). A few very red objects could be at larger redshifts.