The central ray of each composite illumination beam is directed to be {HEAD ~ incident} upon {TAIL ~ a separate preselected location} on the subreflector 
The central ray of each composite illumination beam is directed to be { HEAD ~ incident } upon { TAIL ~ a separate preselected location } on the subreflector
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH