than that of 60[$\,\mathrm{\mu m}$]{}. We also notice two clear outliers, NGC 247 and NGC 300. They have much larger Scanpi flux densities at the same catalogue flux density level than other objects. In the left panel of Figure \[fig:scanpi\], we show the scan data of NGC 247, an extreme example of extended source. In such case, we use zero-point flux density. But it is necessary to manually adjust the background fitting region (especially the exclusion region) to make sure the background is not overestimated.

We estimate the flux density uncertainties based on background rms. It is sensitive to the background fitting region we choose, and the quality of background fitting. Usually smaller background fitting region gives a better background solution (by avoiding close neighbours and large scale fluctuations) and leads to smaller rms. However, the Scanpi data points are not totally independent of each other due to over-sampling. If the background fitting region is too small, the rms will be underestimated. We keep the background region larger than 12 arcmin to minimize such problem. However it is difficult to quantify the impact of background subtraction to the flux density uncertainty we get. We use flux densities with signal-to-noise ratio 