sources themselves are readily computed from the images, and are added to the RMS maps in quadrature. Where available, the exposure time maps were used to convert the images, in electrons per second, to electrons for this computation. Otherwise, the median exposure time was used instead.

A “segmentation vector" was then created which, for each pixel, lists the IDs of objects which contribute light to that pixel. This was done by stacking the Kron ellipses (semi-major axis equal to $2 \times \mathtt{A\_IMAGE} \times
\mathtt{KRON\_RADIUS}$ from <span style="font-variant:small-caps;">SExtractor</span>) of every object in the image. Ellipses are readily generated from the `CXX_IMAGE`, `CYY_IMAGE` and `CXY_IMAGE` ellipse parameters. Neighbours which overlap with a given object are then the unique set of IDs within this object’s Kron ellipse. For each object, a cutout large enough to contain all overlapping neighbours was created from both the image and the total noise map. Overlapping neighbours are fit simultaneously with the object in question. Bad pixels and non-overlapping neighbours were masked by supplying GALFIT with a mask image where bad pixels and pixels within the Kron ellipses of irrelevant sources were given a value of 1, with all other pixels 0. Finally, a PSF image was created 