Accuracy of electrostatic lens computations with FOFEM

The increased speed of personal computers enables fast computation of rotationally symmetric electrostatic lenses with the first-order finite element method in meshes with a large number of mesh points. In order to produce an estimate of accuracy of the computed potential, we propose a simple procedure based on doubling the number of mesh points in each coordinate. In this way, we can produce for the lower-density mesh at each point the information about error of the potential and visualize the sources of the computation errors.