ELECTRON MICROSCOPY 



lo 



Cc/f 

 0-9 



0-8 



0-7 



6 



..^l I 



i 



j ^ 



OOI 002 003 004 005 006 O07 008 O 09 OIO 



\2 



Vr/(NI)^ 



Fig. 5. The variation of the ratio of the chromatic constant (Cc) to the focal length with the excita- 

 tion parameter Vr/(Niy. 



of focus of the instrument. The value of 

 Cc/f is given by the curve of Figure 5 as a 

 function of the excitation parameter. 



To ensui'e no limitation in resolving power 

 or contrast, the variation in focal length (5/) 

 due to ripple on the electron accelerating 

 voltage or lens current supplies must be kept 

 below one quarter the depth of focus of the 

 instrument: 



5/ < 0.7 dVX 



This requires a voltage and current sta- 

 bility meeting the requirement: 



dV/V - 25/// < 0.7 dyxCc 



For a lens near the minimum focal length 

 (Cc '^ 0.4 cm) and for a resolving power 

 near the optimum ('~2-4 A), the required 

 stabilities are in the region of 2 or 3 parts in 

 a million (Haine, 1960). The magnetic lens 

 has the peculiar property of rotating the 

 image with respect to the object. 



Astigmatism arises in objective lenses as 

 a result of very small departures from axial 

 symmetry of the pole piece bores or faces. 

 Only the elliptical component of asymmetry 

 is of significance. To an adequate approxima- 

 tion the astigmatic distance between the 

 tangential and sagittal foci (za) is given by 

 the expression: 



Za = 1005(2 + 3 S/D)Vr/iNiy (7) 



where 5 is the departure from symmetry. 



For the astigmatism not to limit the re- 

 solving power, Za should be small compared 

 with the depth of focus. To achieve the nec- 

 essary symmetry tolerance, which may be a 

 few millionths of an inch for optimum re- 

 solving power, represents an almost impossi- 

 ble mechanical engineering task. Fortun- 

 ately, it is possible to correct a small degree 

 of residual astigmatism by the inclusion of a 

 weak cylindrical lens of variable power and 

 orientation. The progress of correction has 

 been discussed in detail by Haine and IMul- 

 vey (1954). 



Design Considerations 



The number of stages of magnification in- 

 cluded in the microscope is dependent on the 

 maximum magnification and the range of 

 magnification required. A fairly definite op- 

 timum of three stages can be deduced, giv- 

 ing two stages following the objective. The 

 various factors affecting the choice of stage 

 length and the focal length of the various 

 lenses include the maximum and range of 

 magnification, the minimum being limited 

 by image distortion. The practical design of 

 a lens for given pole piece geometry must 

 ensure an adequate magnetic circuit, ade- 

 quate heat dissipation from the excitation 

 coil and a design which can be manufactured 

 within very close symmetry tolerances. The 

 magnetic design is discussed by Mulvey 



152 



