A Method for the Measurement of Spherical Aberration of an Electrostatic Electron Lens

A relatively simple method for the experimental determination of spherical aberration of electrostatic electron lenses is described. This method uses an effective point source of electrons at the center of deflection of a magnetic deflection yoke; the yoke deflects the beam through any desired aperture angle. From the measurement of the displacement of the electron beam in the Gaussian image plane as a function of the aperture angle, the coefficient of spherical aberration can be calculated. The value for this coefficient obtained from the measurement of a unipotential lens and that calculated for this lens by numerical integration are compared; the agreement is within twelve percent. An effective method for the detection of axial astigmatism (which is due to an ellipticity of the lens field) of a lens is described. Diagrams and photographs are given which qualitatively substantiate the method.