2 Observations and data reduction

The new 21-cm line observations of NGC 3310 were obtained with the WSRT as a part of WHISP (Westerbork H I Survey of Spiral and Irregular Galaxies) (Swaters et al. 2001). Due to telescope maintainance only 27 interferometers were available during these observations.

The main observational parameters are listed in Table 2, where "A'' denotes the WHISP observations, "B'' the observations obtained by Mulder et al. (1995) and "C'' the combination of "A'' and "B'' (described below). The combined observations were smoothed to $29\hbox{$.\!\!^{\prime\prime}$ }9\times30\hbox{$.\!\!^{\prime\prime}$ }7$ and $60\hbox{$.\!\!^{\prime\prime}$ }9\times63\hbox{$.\!\!^{\prime\prime}$ }0$ to study the extended, low surface brightness structures.

The map of the radio continuum was obtained from a linear fit of a baseline over the line free channels on either side of the band (28 channels in total) and was subsequently subtracted from the datacube. The resulting line maps were cleaned to correct for instrumental effects like sidelobes, grating rings and offset baselevels. The search area was determined for each channel map separately, in some cases after smoothing to 90 $^{\prime \prime }$ to identify the regions containing extended emission. After cleaning, the new data and those of Mulder et al. (1995) were combined.

The H I channel maps are shown in Fig. 4. The contours show the distribution of H I intensity at 30 $^{\prime \prime }$ resolution. The extended emission is shown at 60 $^{\prime \prime }$ resolution by the shaded areas. The optical picture and the "dirty'' 30 $^{\prime \prime }$ radio continuum map are also shown.