▁Sk ill fully ▁used , ▁these ▁a pert ures ▁allow ▁remark ably ▁precise ▁and ▁detailed ▁study ▁of ▁the ▁defect s ▁in ▁cry st als . ▁ ▁The ▁electron - opt ical ▁system ▁also ▁includes ▁def lect ors ▁and ▁st ig mat ors , ▁usually ▁made ▁of ▁small ▁elect romagnet s . ▁Un like ▁the ▁l enses , ▁the ▁magnetic ▁fields ▁produced ▁by ▁the ▁def lect ors ▁are ▁orient ed ▁primarily ▁to ▁def lect ▁the ▁beam ▁and ▁not ▁to ▁focus ▁it . ▁The ▁def lect ors ▁allow ▁the ▁position ▁and ▁angle ▁of ▁the ▁beam ▁at ▁the ▁sample ▁position ▁to ▁be ▁independently ▁controlled ▁( as ▁is ▁essential ▁for ▁S TE M ) ▁and ▁also ▁ensure ▁that ▁the ▁be ams ▁remain ▁near ▁the ▁low - ab err ation ▁cent ers ▁of ▁every ▁l ens ▁in ▁the ▁l ens ▁stack s . ▁The ▁st ig mat ors ▁provide ▁an ▁auxili ary ▁fine ▁foc using , ▁compens ating ▁for ▁slight ▁imper fe ctions ▁and ▁aber r ations ▁that ▁cause ▁ast ig mat ism — a ▁l ens ▁having ▁a ▁different ▁foc al ▁strength ▁in ▁different ▁directions . ▁ ▁Typ ically ▁a ▁T EM ▁consists ▁of ▁three ▁stages ▁of ▁l ens ing . ▁The ▁stages ▁are ▁the ▁cond enser ▁l enses , ▁the ▁objective ▁l enses , ▁and ▁the ▁project or ▁l enses . ▁The ▁cond enser ▁l enses ▁are ▁responsible ▁for ▁primary ▁beam ▁formation , ▁while ▁the ▁objective ▁l enses ▁focus ▁the ▁beam ▁that ▁comes ▁through ▁the ▁sample ▁itself ▁( in ▁S TE M ▁sc anning ▁mode ,