The resonators secondary and primary mirrors are adjusted into a parallel orientation which gives a series of {HEAD ~ circles} in {TAIL ~ both laser beams} 
in
The resonators secondary and primary mirrors are adjusted into a parallel orientation which gives a series of { HEAD ~ circles } in { TAIL ~ both laser beams }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL