A comparison of experimental and theoretical C_s values for some probe-forming lenses

The shadow projection method has been used to measure the spherical aberration coefficient C_s of a number of asymmetric probe-forming lenses. The results have been compared with values computed from the experimentally measured axial magnetic field distributions and also with values obtained from Liebmann's published data. The agreement between the measured and computed C_s values is good and shows Liebmann's data to be of use, although limited, for the design of weak probe forming lenses. A modified `band' of values of C_s is also presented.