A sub-beam aperture array for forming a plurality of sub-beams from one or more charged particle beams. The sub-beam aperture array comprises one or more beam areas, each beam area comprising a plurality of sub-beam apertures arranged in a non-regular hexagonal pattern, the sub-beam apertures arranged so that, when projected in a first direction onto a line parallel to a second direction, the sub-beam apertures are uniformly spaced along the line, and wherein the first direction is different from the second direction. The system further comprises a beamlet aperture array with a plurality of beamlet apertures arranged in one or more groups. The beamlet aperture array is arranged to receive the sub-beams and form a plurality of beamlets at the locations of the beamlet apertures of the beamlet array.