Properties of two-cylinder electron lenses

The focusing and primary aberration properties of a range of two-cylinder lenses have been determined by trajectory computation. The resistance network analogue was used to obtain the lens fields and the ray equation was integrated numerically by digital computer. Results for the focal lengths, principal plane positions and spherical aberration constants are presented graphically for a large range of geometrical and potential variations. Comparison of spherical aberrations for the different geometries is made, and it is concluded that diameter ratios of unity or less, i.e. the high voltage cylinder of diameter equal to or less than the low voltage cylinder, give the best performance. In some circumstances a diameter ratio as low as 1/3 is found to be optimum. The use of diameter ratio greater than unity appears to be undesirable.