Charged particle optical systems having therein means for correcting aberrations

A charged particle optical system, such for example as an energy analyzer, a mass analyzer or a lens system, the system having

• means, on the optical axis of the system, defining a source of charged particles,
• means, on said optical axis, defining a desired image position, and
• means for defining the path of a beam of substantially monoenergetic particles from said source to said desired image position,
• the system being subject to the aberration in which the trajectories of the particles emitted from the source at relatively large angles to the axis are brought to a focus on the same axis nearer to or further from the source than the trajectories of particles emitted at relatively small angles to the axis,
• the system having a plurality of electrically-insulated corrector electrodes disposed in spaced-apart relationship across the said path of the beam thereby dividing the beam into separate portions and which electrodes, when suitably biassed, so deflect the beam portions as to cause the beam portions to intersect the optical axis at, or closer to, the desired image position, thereby reducing the aberration.