To irradiate a target with a beam of energetic electrically charged particles, the beam is formed and imaged onto a target, where it generates a pattern image composed of pixels. For a pattern which comprises a primary pattern region to be written with a predetermined primary feature size and a secondary pattern region which is composed of structure features capable of being written with a secondary feature size, larger than the primary feature size. The structure features of the primary pattern region are written by exposing a plurality of exposure spots on grid positions of a first exposure grid; the structure features in the secondary pattern region are written by exposing a plurality of exposure spots on grid positions of a second exposure grid according to a second arrangement which is coarser that the regular arrangement of the first exposure grid.