Charged particle energy analysers enabling simultaneous high transmission and energy resolution are described. The analysers have an electrode structure (11) comprising coaxial inner and outer electrodes (14, 15) having inner and outer electrode surfaces (IS, OS) respectively. The inner and outer electrode surfaces are defined, at least in part, by spheroidal surfaces having meridonal planes of symmetry orthogonal to a longitudinal axis of the electrode structure (11). The inner and outer electrode surfaces are generated by rotation, about the longitudinal axis, of arcs of two non-concentric circles having different radii R2 and R1 respectively, R2 being greater than R1. The distance of the outer electrode surface from the longitudinal axis in the respective meridonal plane is R01 and the distance of the inner electrode surface from the longitudinal axis in the respective plane is R02 and R1, R2, R01 and R02 have a defined relationship.