An ion mirror comprising: an ion entrance electrode section at the ion entrance to the ion mirror; an energy focussing electrode section for reflecting ions back along a longitudinal axis towards said ion entrance; a spatial focussing electrode section arranged between the ion entrance electrode section and the energy focussing electrode section for spatially focussing the ions; one or more DC voltage supply configured to apply different DC voltages to the ion entrance electrode section, the spatial focussing electrode section and the energy focussing electrode section, and to apply a DC potential to the ion entrance electrode section that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the energy focussing electrode section; wherein at least two first transition electrodes are arranged between said ion entrance electrode section and said spatial focussing electrode section at different positions along said longitudinal axis, wherein said one or more DC voltage supply is configured to apply a different DC potential to each of the first transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the spatial focussing section to the ion entrance section; and wherein at least two second transition electrodes are arranged between said energy focussing electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply DC potentials to said at least two second transition electrodes that are intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the ion entrance electrode section, and to apply different DC potentials to each of the second transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the spatial focussing section to the energy focussing electrode section. an ion entrance electrode section at the ion entrance to the ion mirror; an energy focussing electrode section for reflecting ions back along a longitudinal axis towards said ion entrance; a spatial focussing electrode section arranged between the ion entrance electrode section and the energy focussing electrode section for spatially focussing the ions; one or more DC voltage supply configured to apply different DC voltages to the ion entrance electrode section, the spatial focussing electrode section and the energy focussing electrode section, and to apply a DC potential to the ion entrance electrode section that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the energy focussing electrode section; wherein at least two first transition electrodes are arranged between said ion entrance electrode section and said spatial focussing electrode section at different positions along said longitudinal axis, wherein said one or more DC voltage supply is configured to apply a different DC potential to each of the first transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the spatial focussing section to the ion entrance section; and wherein at least two second transition electrodes are arranged between said energy focussing electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply DC potentials to said at least two second transition electrodes that are intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the ion entrance electrode section, and to apply different DC potentials to each of the second transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the spatial focussing section to the energy focussing electrode section.