Towards the goal of recording and analysing the elastically filtered intensity of 4–50 keV electrons within the two-dimensional reflection rocking curves (convergent-beam RHEED patterns), preliminary experiments with a one-dimensional serial energy filter have been performed. Zero-loss and plasmon-loss filtered linescans through the specular disc of a CB-RHEED pattern from the Si(001)-2 × 1 surface at 4 kV are shown and compared with dynamical calculations. Energy-filtered detection of CB-RHEED patterns is necessary for quantitative analysis of the surface potential.