An electron gun, having an electron source, a Wehnelt electrode, and acceleration electrodes, includes a control device for changing a field distribution formed by a first acceleration electrode of the acceleration electrodes to control characteristics of a final cross-over formed, by electrons from a first cross-over, at the final stage of the electric gun. The first laser device has a reflecting means in an area which is optically connected to the second laser device, the reflecting arrangement being designed for reflecting radiation generated by the second laser device. The anchoring portion preferably includes an inner tension ring attached to an inner edge to define the opening and a tension ring attached to an outer edge. At another moment, the handle exerts a second force on the engaging member thereby causing the engaging member to move away from the central member, thereby unlocking the binding assembly.