ABSTRACT
The near-axis aberrations are considered for electrostatic systems that are axially symmetric except for the presence of a localized potential defect near the edge of an aperture. When a beam fills the aperture the resulting aberrations cannot be characterized in terms of coefficients, and so a new characterization of the aberrations is defined. Numerical evaluations are carried out for a representative type of potential defect. Relationships are presented that enable the increase in spot size of a focussed beam to be deduced from the measured or estimated mean deflection of the spot.
