▁as ▁deviation ▁from ▁the ▁radial ▁symmetry ▁of ▁the ▁magnetic ▁l ens ▁causes ▁aber r ations ▁such ▁as ▁ast ig mat ism , ▁and ▁w ors ens ▁spher ical ▁and ▁chrom atic ▁aber r ation . ▁Electron ▁l enses ▁are ▁manufact ured ▁from ▁iron , ▁iron - c ob alt ▁or ▁nick el ▁c ob alt ▁allo ys , ▁such ▁as ▁perm al loy . ▁These ▁are ▁selected ▁for ▁their ▁magnetic ▁properties , ▁such ▁as ▁magnetic ▁s atur ation , ▁h yst er esis ▁and ▁per me ability . ▁ ▁The ▁components ▁include ▁the ▁y oke , ▁the ▁magnetic ▁co il , ▁the ▁pol es , ▁the ▁pole pie ce , ▁and ▁the ▁external ▁control ▁circuit ry . ▁The ▁pole ▁piece ▁must ▁be ▁manufact ured ▁in ▁a ▁very ▁symmet rical ▁manner , ▁as ▁this ▁provides ▁the ▁boundary ▁conditions ▁for ▁the ▁magnetic ▁field ▁that ▁forms ▁the ▁l ens . ▁Imper fe ctions ▁in ▁the ▁manufact ure ▁of ▁the ▁pole ▁piece ▁can ▁indu ce ▁severe ▁dist ort ions ▁in ▁the ▁magnetic ▁field ▁symmetry , ▁which ▁indu ce ▁dist ort ions ▁that ▁will ▁ultimately ▁limit ▁the ▁l enses ' ▁ability ▁to ▁reproduce ▁the ▁object ▁plane . ▁The ▁exact ▁dimensions ▁of ▁the ▁gap , ▁pole ▁piece ▁internal ▁diameter ▁and ▁t aper , ▁as ▁well ▁as ▁the ▁overall ▁design ▁of ▁the ▁l ens ▁is ▁often ▁performed ▁by ▁finite ▁element ▁analysis ▁of ▁the ▁magnetic ▁field , ▁whilst ▁considering ▁the ▁thermal ▁and ▁elect rical ▁constraints ▁of ▁the ▁design . ▁ ▁The ▁co ils ▁which ▁produce ▁the ▁magnetic ▁field ▁are ▁located ▁within