The High-Redshift Clusters Occupied by Bent Radio AGN (COBRA) Survey:
  Follow-Up Optical Imaging
  Here we present new red sequence overdensity measurements for 77 fields in
the high-$z$ Clusters Occupied by Bent Radio AGN (COBRA) survey, based on $r$-
and $i$-band imaging taken with Lowell Observatory's Discovery Channel
Telescope. We observe 38 COBRA fields in $r$-band and 90 COBRA fields in
$i$-band. By combining the $r$- and $i$-band photometry with our 3.6$\mu$m and
4.5$\mu$m $Spitzer$ IRAC observations, we identify 39 red sequence cluster
candidates that host a strong overdensity of galaxies when measuring the excess
of red sequence galaxies relative to a background field. We initially treat the
radio host as the cluster center and then determine a new cluster center based
on the surface density of red sequence sources. Using our color selection, we
identify which COBRA cluster candidates have strong red sequence populations.
By removing foreground and background contaminants, we more securely determine
which fields include cluster candidates with a higher significance than our
single-band observations. Additionally, of the 77 fields we analyze with a
redshift estimate, 26 include newly estimated photometric redshifts.
