Top left: zoom onto PAH- and extinction-corrected image of the MWNSC at 4.5 μm. Top right: symmetrised image resulting from folding and median-averaging the four quadrants. Both images in the top have been median-smoothed with a 3-pixel wide box to enhance the large-scale structures. Bottom left: difference between top left and right panels. Bottom right: difference image divided by the uncertainty map resulting from quadratically combining the pixel uncertainties of the images in the top left and right panels. Galactic north is up and Galactic east is to the left. The colour bar in the bottom right panel is in units of standard deviation.