2. The data
2.1. The survey observations
The survey observations with all 3 cameras started in routine mode in December 1995 and were slowed down by technical and meteorological problems in 1997 and 1998. The survey is currently (June 1999) progressing at nominal speed.
Special efforts have been devoted to cover priority regions of high scientific interest, such as the Magellanic Clouds, the Galactic bulge, and giant molecular clouds.
The observing technique is the following: the sky, south of declination , has been divided into 5112 regions named slots . A slot is wide in right ascension, and spans in declination. Two adjacent slots have overlaps in right ascension or declination. A DENIS strip corresponds to the observation of a slot. A given slot may be observed several times (for example if the photometric quality has been detected as insufficient by the data reduction software). Thus several strip numbers can refer to the same slot. A strip is made of 180 individual images (12´12´), with overlaps between consecutive images.
2.2. Point source data reduction
All the images received from La Silla are first processed at the Paris Data Analysis Center (PDAC), where the images are checked and calibrated (Borsenberger 1997); afterwards, the point source extraction (Bertin & Arnouts 1996) and the astrometric and photometric calibrations (Deul et al. 1995) are carried out at the Leiden Data Analysis Center (LDAC). Each strip is processed independently. Analysis of extended sources, carried out at PDAC, is not considered in the present article, which deals only with point sources.
2.3. Data selection
The preliminary database of DENIS point sources released at CDS provides, for 102 strips (as of June 1999), the three-colour information resulting from the reduction pipeline.
The available strips were selected according to their astrometric and photometric quality, in order to form a useful sample of the current DENIS point source catalogue. It is to be noted, however, that this is not a subset of the final catalogue of point sources, because some improvements in the reduction pipeline are still currently being implemented, and this will imply a new reduction of already observed data. These improvements are not, however, of such a nature that they could change the statistical interpretation of the currently released data sets.
Within a strip, association has been done between bands (I, J, ), and overlapping images (with the notable exception of bad quality flagged sources which are not matched and appear duplicated). The positional coincidence is determined by examining the elliptical shape parametrization of each entry. When a source is present several times in a given band, the resulting merged entry in the catalogue carries the simple flux average for the magnitudes, and the weighted average for the coordinates. Eventual overlaps between adjacent (12´ 30o) strips have not been matched in this preliminary release. These elements should be carefully taken into account when working on star counts derived from the released data.
It should also be noted that in the final DENIS catalogue, overlaps between adjacent strips will lead to a better accuracy in the photometry (see Sect. 3.2).
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999