2 Obtaining I-band CCD frames from DENIS

2.1 Characteristics of the DENIS survey

The I-band of the survey is the Gunn-i band at 0.80 $\mu$m. The CCD camera is a Tektronix 1024$\times$1024 pixel array cooled down to liquid nitrogen temperature. Each frame (768 $\times$ 768 pixels) represents 12 square arcminutes with a pixel size of 1 arcsec. The integration time is 9 s. The read-out noise is about 7 e^-. The observing strategy consists in scanning at a constant right ascension on strips of 30 degrees in declination (180 frames per strip) taken in three zones "Equatorial'' from $\delta=+2 \deg$ to $-28\deg$, "Intermediate'' from $\delta=-28 \deg$ to $-58\deg$and "Polar'' from $\delta=-58 \deg$ to $-88\deg$. The overlap between adjacent frames is 1 arcmin on each side (i.e. 2 arcmin in each direction). This strategy aims at covering a wide range of airmasses. Each strip starts and ends with photometric and astrometric calibrations. At the end of some nights a flat fielding is performed directly on the sky during sunrise. The data is archived on DAT cartridges which are send each week to the Paris Data Analysis Center (PDAC) at the Institut d'Astrophysique de Paris for processing.

2.2 Pipeline Lyon-PDAC

A systematic automatic processing of DENIS I-images began in Lyon in February 1996. Flat-fielding and de-biasing are made at PDAC on each genuine frame. For the Lyons processing each I-image is reduced by a factor 4 in size by rebinning pixels 2 $\times$ 2. Our effective pixel size is thus 2 arcseconds. An example of an I-frame is given in Fig. 1.

  

Figure 1: A typical image from the DENIS Survey after the $2 \times 2$ rebinning. The frame is $12' \times 12'$. North is on the bottom side, East is on the left side. Among the four galaxies clearly visible on the top, one (the third from the left) is a new one. This shows that relatively bright galaxies can be discovered

The histogram of pixel intensities is used to calculate the sky background intensity $f_{\rm bg}$.The sky-background level is taken at the maximum intensity of the histogram (i.e. the mode) and the standard deviation $\sigma$ is calculated by symmetrizing the low intensity part of the histogram with respect to the mode. The sources are concentrated in the high intensity part of the histogram.

A threshold is then applied (the threshold level is $f_{\rm bg} + 2.0 \sigma$). Using this procedure (averaging and thresholding) allows a compression factor of 20 to 30, depending on the image contents. All images of a given I-strip are thus compressed, tar'd and automatically transferred to Lyon via ftp. A full strip is stored in 10 to 13 Mbytes. The galaxy extraction is made at Lyon using the program described in the following section.