SED fits of the filament models. Frame a shows the SEDs towards the map centre and the 1-MBB fits to the 4–6 longest wavelength bands. The lower set of symbols and curves corresponds to a model without a point source, and the upper ones to a model with a point source on the LOS towards the filament but at 0.93 pc in front of the filament. The colour temperatures of the 1-MBB fits are quoted in the frame. Frame b shows the ratio of recovered and true peak optical depths (left axis, filled symbols) and the average χ² per data point over the whole map (right axis, open symbols). The x-axis indicates the four models. The square and star symbols are, respectively, for the externally heated model and the model with a point source. The black, blue, and red symbols correspond to fits that extend to shortest wavelengths of 160, 100, and 70 μm, respectively. The error bars show the standard deviation of the optical depth estimates along the filament spine.