82 survey [@Adelman-McCarthy2007], which is $\sim2$ mag deeper.

From the *redMaPPer catalogue* we construct a close galaxy pair sample of 1336 pairs by searching for all photometric galaxies within a physical separation distance $(r_{\mathrm{sep}})$ of $7 \leq r_{\mathrm{sep}} \leq 50$ kpc from the BCGs. Galaxies within this search radius are referred to as companions. The lower limit is imposed since this is the minimum separation distance down to which the SDSS can resolve individual galaxies over our redshift range.

A close pair sample that has only been constructed using photometric information will inevitably suffer from contamination due to line-of-sight projections. Although the contamination in clusters is higher than in the field (including contamination from cluster members themselves), the same techniques can be used to obtain the close pair fraction. In this work we use spectroscopy to correct for this contamination by determining whether the photometrically identified companion galaxies are bound to their host BCGs. In order for close pairs to be considered potential merger candidates, we require the galaxies to have a velocity difference of $\Delta v \leq 300$[kms$^{-1}$]{} [see e.g. @Burbidge1975; @Ellison2013; @Kitzbichler2008]. We have chosen this cut, which is somewhat stricter than many other observational works [e.g. @Lin2004a; 