in appearance to DGSAT I [@martinez-delgado2016]. The raw number of UDGs detected in each cluster is listed in Table \[tab:clusters\]. Figure \[fig:locations\] shows the locations of all selected UDGs within the WFC3 coverage (pink outline) for each cluster core field, along with $15
\times 15$ kpc zoom-ins on select UDGs.

Note the remarkable non-uniform projected spatial distributions of UDGs in some of the clusters, most notably in Abell 2744, MACS 1149 and MACS 0717. Only in Abell S1063 and Abell 370 do the UDGs appear to be evenly distributed around the cluster. In the other clusters, there appears to be many more UDGs on one side than the other. UDGs also appear to avoid the central regions of the clusters. For now though we turn to corrections and simulations needed to understand the physical significance of these effects, if any.

Background correction
---------------------

For each FF cluster, an estimate of the background contamination to the UDG sample is made by computing the physical sizes and absolute surface brightnesses of every XDF source assuming the source lies at the FF cluster redshift. We then apply the UDG cuts described above. By assuming all XDF sources lie at the FF cluster redshift 