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