Disclosed herein is an apparatus comprising: a source of charged particles configured to emit a beam of charged particles along a primary beam axis of the apparatus; a condenser lens configured to cause the beam to concentrate around the primary beam axis; an aperture; a first multi-pole lens; a second multi-pole lens; wherein the first multi-pole lens is downstream with respect to the condenser lens and upstream with respect to the second multi-pole lens; wherein the second multi-pole lens is downstream with respect to the first multi-pole lens and upstream with respect to the aperture.