Optimum adjustment and correction of the Wien filter

The crossed homogeneous electrostatic and magnetic deflection fields of an analyser (Wien filter) have been formed by two condenser plates and two pairs of coils. The optimum operating conditions are shown to require unit magnification, contrary to the view of other authors. By varying the current ratio in the coils, a desired inhomogeneity can be introduced in the magnetic field and with this the second-order angular aberration can be corrected. The possibility of a double-focusing Wien filter is discussed.