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