For maskless irradiating a target with a beam of energetic electrically charged particles using a pattern definition means with a plurality of apertures and imaging the apertures in the pattern definition means onto a target which moves (v) relative to the pattern definition means laterally to the axis, the location of the image is moved along with the target, for a pixel exposure period within which a distance of relative movement of the target is covered which is at least a multiple of the width (w) of the aperture images as measured on the target, and after said pixel exposure period the location of the beam image is changed, which change of location generally compensates the overall movement of the location of the beam image.