Upper image: highest peak of the Faraday depth spectrum in polarization at each spatial pixel. Bottom image: map of the RM value of each peak, shown in red-blue colour scale. Yellow contours are showing Hα intensity on a one degree scale (Finkbeiner 2003; Haffner et al. 2003), given in units of Rayleighs (see Sect. 5). A white box indicates the boundaries of the WSRT 350 MHz image in Fig. 10. A location of the discovered pulsar J081558+461155 (see Sect. 3.3) is shown with a yellow asterisk.