Modified operation of the seven-element electrostatic Chutjian–Kuyatt lens system

We reconsider the properties of the seven-element electrostatic zoom-lens system of Chutjian and Kuyatt [Rev. Sci. Instrum. 50, 347 (1979)] from our experience with a ‘‘visual’’ beam-tracing program, and propose a modified operation of this lens system to achieve simultaneously correct focusing, zero beam angle, and constant target size over the zoom range. Rational function fits to all driving voltages are given. A ray-tracing program for a personal computer is described in the Appendix.