larger than 3 as detections. For detections, we add 10% additional calibration uncertainty [@1984ApJ...278L...1N]. For non-detections, we give 3-$\sigma$ upper limits based on background rms.

There are 15 (at 60[$\,\mathrm{\mu m}$]{}) and 12 (at 100[$\,\mathrm{\mu m}$]{}) galaxies detected with Scanpi but without counterparts in PSC or FSC. We find that most of these objects show complicated background, implying that poor background subtraction is the main reason for PSC/FSC non-detection. The right panel of Figure \[fig:scanpi\] shows the scan data of NGC 5237, as an example. The galaxy itself is relatively weak hence it is buried in the large scale fluctuation of the background when performing blind source finding. We also show the strong impact of background estimation to flux density measurement for such weak objects. The red and orange dashed lines are the estimated background when using fitting region radius of 15 arcmin and 30 arcmin, respectively. The 30 arcmin background does not well represent the local background at the position of galaxy, due to the complex background fluctuation, and will cause significant flux missing if we use zero-point flux density or centre flux density. Similar situation may occur even for a moderately bright object, if it is close to 