2. Observations and data reduction
A region centered on the CFRS 1415+52 field was observed with ISO in a raster pattern such that a constant exposure time per pixel was achieved over the whole CFRS field. Given the small size of this field, it was decided to go as deep as possible with ISOCAM at 6.75µm, partly so that the results could be used as a possible template for the ISOCAM Central Program for deep surveys (Césarsky et al. 1996), as well as for other large surveys.
Eleven individual images were taken in the micro scanning AOT mode (CAM01, a raster of 44 with 8 or 12 readouts per step) with the ISOCAM LW channel ( per pixel) and the LW2 filter (5-8.5µm), leading to a total integration time of 600 sec pixel-1. The micro scanning mode provides the best spatial resolution through superposition of images. The same pixel of the sky was placed in different parts of the camera in order to minimize and detect any systematic effects. The micro scanning AOT technique also allows an accurate flat-field image to be generated and yields a pixel size of 1:005 in the final integrated image. The detection and removal of transients and glitches, integration of images, absolute flux calibration, and source detection were carried out using the method described by Désert et al. (1998). This method has been found to be particularly well adapted to our observational strategy i.e. coadding the eleven images, without redundancy within each image. Special attention was paid to possible error propagation in the flux values. Finally, to further detect and eliminate relatively weak (S/N 3) spurious sources, the data were also reduced with the CIA PRETI software (Aussel et al. 1997 and Starck et al. 1998) and compared with the initial results. However latter software is less adapted to our data since we have generated several images the micro scanning technique, and glitch removing is more difficult. It is beyond the scope of this paper to compare the two data reduction procedures in order to estimate the photometric accuracy, and this question is addressed elsewhere (Desert et al., 1998). On the other hand, sources which have not recovered with PRETI software will not be further considered (Sect. 2.1)
Individual images were carefully registered with each other in order to optimize the image quality of the brightest compact objects (mostly stars). However, this registration is limited by the presence of glitches (defaults and cosmic rays). In order to obtain the best possible accuracy, an iterative procedure was adopted in which shifts were determined from the average of the 3-5 highest S/N sources in each of the 11 individual images and the composite image, the frames were then offset and combined to form a new composite image, and the procedure repeated three times. As demonstrated in Fig. 1, this results in a significant improvement of the shapes and FWHM of the sources. The final image (Fig. 2) of the whole ISO field has a resolution equivalent to a median FWHM (calculated with DAOPHOT under IRAF).
The precise location of the ISOCAM image relative to deep CFRS and I images of the 1415+52 field was determined from the six brightest ISOCAM LW2 sources (5 stars and the z = 0.216 galaxy CFRS14.1157).
Fig. 3 shows the 5-8.5 micron flux distribution corrected for aperture effects (1.4, for details, see Désert et al. 1998 and references therein).At the S/N 4 limit, our number counts (21 per 100 arcmin2) are comparable to those of the Deep ISOCAM Survey ( counts per arcmin2 for sources with a flux 250 µJy, Césarsky et al. 1998, in prep.). The validity of detection at S/N 4 is confirmed by studies in Lockman-Hole Deep Survey(Césarsky et al. 1998, in prep.), which show that sources with S/N in individual frames are confirmed in 95% of cases, in the final integrated image (S/N 10)
2.1. ISO source catalogue
Classifications and verification of the source detections were made on the basis of S/N and the repeatability of the detections in three independent combinations of the 11 individual images (for details about the source detection, repeatability and classification, see Désert et al. 1998). The repeatability test is based on the redundancy factor, which is the number of times that the sky pixel was seen by different pixels on the camera. The software built three independent projection subrasters, and for each source candidate the flux and error are measured at the same position in each subrasters. The quality factor is based on flux measurements and varies from the best confidence index (=4) to the worse confidence level (=0, see eqs. 7 to 11 of Désert et al.,1998). We have considered only sources with confidence level higher than 3, which means that the final source flux and source fluxes in subrasters are within 3 (2 in the case of 4), where is the error value of the final flux source.
The final catalogued sources were also all detected in the image which was reduced by the second independent analysis (using PRETI). Altogether, 54 sources with S/N 3 met these criteria, but only the subgroup of 23 sources with S/N 4 are considered to be secure detections. The 54 sources are listed in Tables 1 and 2. The first two sections of Table 1 contain 21 ISO sources (catalogues 1 & 2) that have secure optical identifications. The 24 sources in the lower half of Table 1 and the 9 sources in Table 2 have less secure or no optical identifications, respectively.
Table 1. Optical counterparts of ISOCAM LW2 sources.
Table 2. ISOCAM LW2 sources without optical counterparts. Those sources above the line are in catalogue 3 (S/N 4) and those below comprise catalogue 6 (4 S/N 3).
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999