Raw images from the multi-slit narrow-band spectroscopy of NGC 891. The intensity scale is inverted such that bright emission is black. The instrument (OSMOS) has been rotated for 68° so the galactic disk is placed perpendicular to the slits. (a) Slit image overlaid on a snapshot Hα image, taken with the same filter KP1468 as the spectroscopy observations, and with the exposures of 3 × 60 s. The black box indicates the region shown in Fig. 2. (b) Raw spectra taken with the multi-slit mask and the KP1468 Hα narrow-band filter. The image in the leftmost side should be higher-order spectra of the rightmost slits, which are not used in the follow-up analysis. Each rectangle is a narrow-band spectrum with the 60″ slit segment, that traces spatial length in the vertical direction and wavelength spanning the slimmer horizontal direction. The inset panel shows a zoom-in of the central region, where the three prominent emission lines (Hα, [N II]λλ6548, 6583 Å) can be clearly seen as bright vertical lines.