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