2 WENSS

The Westerbork Synthesis Radio Telescope (WSRT) is an east-west array, consisting of fourteen 25 meter dishes (Baars & Hooghoudt 1974). A twelve hour observation is needed to get spatial resolution in all directions. Ten of the dishes have fixed positions and are spaced 144 meters apart. The remaining four are moveable, although their mutual distances are usually kept constant. By observing an object several times with different distances between the fixed and moveable subarray, different baselines are obtained and the uv-coverage improves, yielding an improved synthesised antenna pattern.

The Westerbork Northern Sky Survey (WENSS) is a 325 MHz continuum survey of the sky above declination +30$^\circ$ (Rengelink et al. 1997; de Bruyn et al., in preparation). This area was surveyed using a mosaicing technique. Each field was observed 18 times for 20 seconds spread over 12 hours. Each mosaic was observed on six days with a different spacing between the fixed and moveable subarray to get a uniform spatial distribution. These observations were spread over periods of weeks to years. The resulting flux densities are averaged over all these observations.

The WENSS beam size is 54 $^{\prime \prime }$$\times$ 54 $^{\prime \prime }$  $\mbox{$\mathrm{cosec}$ }~\delta$(FWHM). The final maps have pixel sizes of 21.09 $^{\prime \prime }$. When a pixel was found with a flux density above five times the local noise level, a two-dimensional Gaussian was fitted to its surroundings. The coordinates of the centroid of the fit, the maximum (peak) flux density and the flux density integrated over the fit were added to the source list. Extra flags in the catalog mark multiple and extended sources. For a point source the peak flux density equals the flux density integrated over the beam. Only the peak flux densities are used in this analysis, since pulsars are intrinsically point sources. Bright sources have positional errors of 1.5 $^{\prime \prime }$, weaker sources at lower declinations have errors up to about 10 $^{\prime \prime }$. On average the uncertainty is 5 $^{\prime \prime }$.

Figure 1: Pulsar positions and WENSS source densities. Filled circles mark the positions of the pulsars which are correlated with a WENSS source. Open circles indicate the pulsars, with a marginal detection in the WENSS. Confused pulsars are marked with a star. Pulsars with no WENSS counterpart are indicated with a plus. The grey-scale indicates the WENSS source density (white is low density). The polar cap region has been surveyed with a larger bandwidth and has a higher source density. A small region near Cygnus A (RA 20h, Dec 40$^\circ$) is blanked in the WENSS. The solid line indicates the Galactic plane

The total bandwidth was 5 MHz. For most regions of the sky the detection limit was between 15 and 25 mJy (five times the local noise level). The polar cap (declination > +75$^\circ$) was surveyed with a larger bandwidth and the detection limit for this area was about 10 - 12 mJy. The WENSS source catalog contains 229420 sources (de Bruyn et al. 1998). A total of 18186 of these are located in the polar cap area. Figure 1 shows the density of sources in the WENSS area.