Illustration of our polynomial sky subtraction, with our SPIRE 250 μm map of NGC 6946, over a 45′ × 30′ portion of the map. As can be seen, NGC 6946 suffers from a high degree of contamination from foreground Galactic cirrus. Photometric master aperture ellipse, as determined by CAAPR, indicated by solid green line; background annulus is demarked by dashed green lines. Left: original, un-subtracted map. Central: best-fit 2-dimensional 5th order polynomial sky model used to remove large-scale emission, right: map after undergoing polynomial sky subtraction (displayed using same colour scale as in the left panel); note that background has been appreciably flattened.