{HEAD ~ an objective lens} for focusing {TAIL ~ the electron beam} emitted from said electron source upon the object using an output from the height detector 
for focusing
{ HEAD ~ an objective lens } for focusing { TAIL ~ the electron beam } emitted from said electron source upon the object using an output from the height detector
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH