40 



004 



OOI 



ELECTRON OPTICS 



0025 Vr /(Nl)^ 



Niy/vv 



Fig. 6. The image rotation angle in radius plotted as a function of NI/\/V\- and Vr/iNI)^ (on top 

 scale). 



(1953). Permissible thermal loading of the 

 coil varies from about 800 ampere-turns per 

 square centimeter of cross-sectional area of 

 coil for a lens without water coohng, to 1200 

 ampere-turns per sq cm for a carefully de- 

 signed water-cooled coil with no interleaving 

 paper. The values are fairly independent of 

 the wire size used, which may be chosen to 

 give a coil impedance most suitable for the 

 current supplies. 



It was at one time thought that the great 

 precision of symmetry required in the objec- 

 tive lens required the pole pieces to be manu- 

 factured separately from the main iron 

 shroud and fitted precisely within its bore. 

 That this is not so, and in fact leads to un- 

 necessary complication, has been shown by 



Haine (1954). Although some manufacturers 

 still utilize separate pole pieces, the tendency 

 is toward the simpler lens made from two 

 pieces of iron. 



Electrostatic Lenses 



The data for electrostatic lenses cannot be 

 expressed by such simple means as for mag- 

 netic lenses. The variation of focal length 

 and spherical aberration with the dimensions 

 for three aperture unipotential lenses of spac- 

 ing (*S), central electrode aperture diameter 

 D and thickness T are given in Figures 7 and 

 8. 



The objective cannot be immersed in the 

 electrostatic field and it will be seen that the 

 spherical aberration is about an order of ten 

 greater than for the magnetic lens. 



153 



