Fig. 6

The family of electron rays simulated in the MEM system of Fig. 1, incident on the \( t=15 \) min surface as shown in Figs. 2 and 5. a Electron paths close to the surface demonstrate how changes in the height, e.g. discontinuities, can cause electron paths to overlap in the returning beam. b Apparent straight line paths of the exiting electrons traced back to the virtual image plane \( z=\Delta f+4{L}_{\mathrm{M}}/3, \) 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 changes in the surface height function create regions where electron paths overlap (both in the returning beam and near the virtual imaging plane), which are evident as bright caustic rings in the simulated image (Fig. 3)