of background estimation. If the background is poorly determined, especially when there are blended sources or foreground cirrus structures, the estimated flux density will be dramatically wrong. The centre flux density is the integration of averaged scan within the central $\pm2.5$ arcmin (60[$\,\mathrm{\mu m}$]{}) or $\pm4$ arcmin (100[$\,\mathrm{\mu m}$]{}) region. It is a compromise of model flux density and zero-point flux density. It gives a reasonable alternative estimation for point-like sources when the template fitting fails. It is less dependant on background than the zero-point flux density but not as useful when the source extends beyond the integration region. For each galaxy, we manually choose the optimal method.

![image](fig/iras60.pdf){width="8cm"} ![image](fig/iras100.pdf){width="8cm"}

![image](fig/scanpi_ngc247.pdf){width="8cm"} ![image](fig/scanpi_ngc5237.pdf){width="8cm"}

Figure \[fig:fir\_flux\] shows a comparison between the flux density we measured with Scanpi and that from PSC or FSC. It is clear that the Scanpi flux density is systematically larger than the catalogue one. The offset is larger when the source is brighter. It is consistent with our expectation. The brighter sources are usually more massive galaxies, more likely to be extended sources. The offset is stronger in 60[$\,\mathrm{\mu m}$]{} than in 100[$\,\mathrm{\mu m}$]{}. It can be understood as the PSF FWHM of 100[$\,\mathrm{\mu m}$]{} is much larger 