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