We have used the OSIRIS narrow band tuneable filter to take a sequence of images of the star-forming spiral NGC 6946 incorporating the full galaxy, spatially, and the wavelength range which takes in the two lines of the [SII] doublet at 671.9 nm and 673.0 nm, spectrally. This effective data cube allows us to determine the intensity ratios of the two lines in HII regions across the disc, and to derive the corresponding in situ electron densities for several hundred regions. We find, as expected, a trend to lower densities at larger galactocentric radii.The technique used, and our initial results, will be presented.