A charged particle filter includes a magnetic deflector having a bore along an axis thereof passing through the magnetic deflector from a sample end to a detector end of the magnetic deflector, and through which bore charged particles pass when in use, the magnetic deflector being formed from two magnets positioned around the bore, with a gap between the two magnets, the two magnets each having a linear central section and two ends, each end forming a curved surface, the curved surface having an aspect ratio defined by a height in a range of between one tenth and ten times the gap between the two magnets, and a width in a range of between one tenth and ten times the gap.