Use an iMac with Thunderbolt as an external display
I should had read the KBase better .... Use your iMac as a display


  Note: Target Display Mode isn’t supported on iMac models with Retina
  display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and
  iMac (Mid 2011 to Mid 2014) support Target Display Mode.