The invention relates to a multiple beam charged particle optical system comprising: a charged particle source for generating a plurality of charged particle beamlets, and charged particle optics for directing the charged particle beamlets from the charged particle source towards a target, wherein each charged particle beamlet defines a beamlet centre line,said charged particle optics comprising one or more electrostatic lens arrays, each comprising two or more array electrodes for generating a plurality of electrostatic lenslets,wherein each lenslet is arranged for focusing a corresponding charged particle beamlet, and wherein each lenslet defines a lenslet optical axis,wherein at least one of said one or more electrostatic lens arrays comprises one or more off-axis electrostatic lenslets, wherein the beamlet centre line of the corresponding charged particle beamlet passes through said off-axis electrostatic lenslet at a distance from its lenslet optical axis.