For proper functioning of the electron gun, it is necessary that the beam be perfectly adjusted with respect to the optical axes of the accelerating electrical lens and the magnetic focusing lens. This can be done by applying a magnetic field of some specific radial direction and strength perpendicular to the optical axis before the focusing lens. This is usually realized by a simple correction system consisting of two pairs of coils. By adjusting the currents in these coils any required correcting field can be produced.