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