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
(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
54
(FWHM). The final maps have pixel sizes of 21.09
.
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
,
weaker sources at lower declinations have errors up
to about 10
.
On average the uncertainty is 5
.
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)
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.
Copyright The European Southern Observatory (ESO)