L1 gop.gopop1(if(gop.pidx==1,gop.L,0)) "Optical path length, first ray"
L4 gop.gopop1(if(gop.pidx==4,gop.L,0)) "Optical path length, fourth ray"
dL L4-L1 "Optical path difference"
dnu dL/P*nu "Beat frequency"
SF dnu/Omega "Scale factor"