A charged particle filter comprises a magnetic deflector and an outer shield. The magnetic deflector has a bore along an axis thereof passing through the deflector from a specimen end to a detector end of the deflector and through which charged particles pass when in use. The deflector is formed from one or more magnets positioned around the bore in a Halbach configuration thereby generating a relatively high magnetic field strength within the bore and a relatively low magnetic field strength outside of the deflector. The deflector has a geometry defining an outer surface and an inner surface, wherein each of the outer and inner surfaces of the deflector taper towards the axis as a respective function of distance in the specimen direction along the axis. The outer shield is formed from a soft magnetic material surrounding the magnet deflector on an outer surface side of the deflector and having a projecting portion which extends in the specimen direction with respect to the magnetic deflector from the specimen end of the deflector.