A lens may be provided to focus {HEAD ~ both beams} on a small hole in {TAIL ~ the mirror} 
A lens may be provided to focus { HEAD ~ both beams } on a small hole in { TAIL ~ the mirror }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL