Left: VISIR/VLT mid-IR monochromatic image at 8.59 μm (Lagadec et al. 2011). Right: radial profiles at 8.59 μm, 11.85 μm, and 12.81 μm are extracted and overplotted with the modelled profiles as described in Sect. 6.2. The size and position of each shell is indicated in light blue. The point spread function (PSF) at 11.85 μm is also overplotted for reference, revealing the hot inner shell. The PSF becomes broader at longer wavelengths.