Illustration of the correction curve method. Top: CO fluxes of Serpens SMM1 measured over 5 × 5 array divided by the central spaxel fluxes are plotted versus wavelengths. PACS PSF for a point source is shown in green. CO emission is clearly extended for this sources. Middle: best signal-to-noise measurements are used to make a fit to the data and derive the wavelength-dependent correction factors. Bottom: CO fluxes measured over 5 × 5 array are divided by central spaxel measurements corrected for the extended emission using calculated correction factors are plotted versus wavelength. Accuracy longward ~100 μm is better than 10%, whereas the accuracy is ~30% for short-wavelength lines (≤100 μm).