not be entirely encompassed by the FOF source regions. If this happens, though, it may imply the presence of extended emission embedded in a larger background region (for whatever reason) and the source is tentatively classified with a flag as extended. We remark, however, that this procedure may introduce several false candidates if the PSF shape is not completely under control. Eventually, these cases may be visually inspected for final inclusion in the extended source list. Finally, as a very last step, extended sources found within a larger, surrounding extended source are merged into it and considered part of the largest one.

Source regions and aperture photometry\[photometry\]
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The definition of a FOF detected source region depends on the linking length, which is the only parameter of FOF. The linking length is defined as the median distance between occupied pixels in a pure background image, and therefore it depends on the background intensity only. An estimate of the linking length in units of pixel size is given by $\sqrt{Bkg^{-1}}$, where $Bkg$ is in units of $photon/pixel$. Clearly the linking length depends on the position on the image, therefore we defined a linking length map according to the background map. Using 