Fig. 7

The family of electron rays simulated in the MEM system of Fig. 1, incident on the \( t=20 \) min surface as shown in Fig. 5. a Electron paths close to the surface demonstrate how the surface can act as an electron lens to focus the electrons in the returning beam. b Apparent straight line paths of the exiting electrons traced back to the virtual image plane \( z=\Delta f+4{L}_M/3, \) for defocus \( \Delta f \) controlled by the magnetic part of the objective lens. The \( x \) positions of the discontinuities in the surface height profile are indicated by vertical dashed lines. The \( x \) positions of the rays have been multiplied by \( 3/2 \) to remove the demagnification of the anode aperture as discussed in [26]. Note that the focusing of the electron beam in the returning trajectories creates a bright caustic central spot on the virtual image plane and the simulated image (Fig. 3)