3. Data acquisition
The HDF field is well suited for mid-infrared deep observations. It was selected for its low optical extinction, low HI column density and, most important fact for ISOCAM observations, low cirrus emission in the IRAS 100 µm maps (Williams et al. 1996). Moreover, it sits at high galactic latitude, and contains therefore only a few stars. Similar criteria were used to select fields for other ISOCAM deep surveys such as the IDSPCO program (Cesarsky & Elbaz 1996) or the DEEPPGPQ program (Taniguchi et al. 1997).
Table 1 describes the observation parameters for both filters. Each consist of three 8x8 rasters, which are partly overlapping. In both cases, the area covered is wider that the HDF field, and even contains a large part of the flanking field for the LW3 observation, which was done with a larger pixel field of view (PFOV) and a greater raster step than the LW2 observation. If the detector had been ideal, i. e. with no memory effect and insensitive to cosmic rays, a very good sensitivity would be reached:
where is the noise per readout for each ISOCAM pixel (it is therefore equal to the readout noise plus the photon noise, which is dominated in the case of faint sources by the background emission), and is the number of exposures. One can easily compute the ideal sensitivity of the ISO-HDF if one computes for each pixel: this is easily done by fitting a Gaussian distribution on the histogram of the each pixel, after having filtered it of all the low frequency components, because all the non ideal behaviour of ISOCAM occurs mostly at low frequency. At a 5 level, one could reach 15 µJy in the LW3 band and 5 µJy in the LW2 band. The observation was designed to be as deep as possible, and with a good spatial resolution in order to be able to identify with confidence the optical counterparts of ISOCAM sources. The observations were therefore made using the microscan mode, that is by moving the camera by fraction of pixels between two raster steps. With a step of 5 arcsec and a PFOV of 3 arcsec, the final resolution of the ISOCAM map should reach 1 arcsec in LW2, and 3 arcsec in LW3.
Table 1. ISO-HDF observation parameters. The main parameters for each raster and for the whole observation are listed
Unfortunately, the long wavelength detector (LW) of ISOCAM is very sensitive to cosmic rays due to its thickness, and shows a transient behaviour. These two main drawbacks alter its efficiency in detecting faint sources. This is the reason why the PRETI method was developed.
© European Southern Observatory (ESO) 1999
Online publication: February 22, 1999