0.7$, $H_0 = 70~\mathrm{km}~\mathrm{s}^{-1}~\mathrm{Mpc}^{-1}$. All magnitudes are in the AB system. Galactic extinction corrections from the [@schlafly2011] extinction maps were applied to all colours and magnitudes.[^1]

Data {#sec:data}
====

The *HST* FF program has produced the deepest images to date of galaxy clusters and gravitationally lensed galaxies for six high-magnification clusters— Abell 2744, MACSJ0416.1$-$2403, MACSJ0717.5$+$3745, MACSJ1149.5$+$2223, Abell S1063 (also known as RXCJ2248.7$-$4431) and Abell 370— along with six corresponding parallel “blank" fields offset ${\sim}6\arcmin$ from each cluster [@lotz2017]. These clusters were chosen for their known high lensing strengths, low sky backgrounds and Galactic extinctions, in addition to observability with *HST*, *Spitzer* and ground-based facilities [@lotz2017]. Selecting galaxy clusters for their lensing strength will end up selecting extremely massive, merging clusters, as the merger stretches the lensing critical curves between the various components resulting in relatively large areas subject to high magnification [@redlich2012; @diego2016]. The cluster properties are summarized in Table \[tab:clusters\]. The coordinates are the cluster centres as defined by their stellar content, that is the location of the brightest cluster galaxy (BCG), or where the cluster is comprised of multiple merging subclusters, the centroid of the BCGs. The diverse and complex morphologies displayed by these clusters is discussed later. 