A single-reflecting time of flight analyser comprising: a pulsed ion source (30, 40, 50); a non-linear ion mirror (32, 42, 52) having a turn-around point; a detector (33, 43, 53); an ion flight axis extending from the pulsed ion source to the detector via the turn-around point of the non-linear ion mirror, the ion flight axis defining a x-direction; and a y-axis defining a y-direction and a z-axis defining a z-direction, the y-axis and the z-axis being mutually orthogonal and orthogonal to the ion flight axis, the pulsed ion source being configured to produce an ion pulse travelling along the ion flight axis, the ion pulse comprising an ion group, the ion group consisting of ions of a single m/z value, the ion group having a lateral spread in y- and z- directions, wherein said lateral spread is determined by an energy spread and a spatial spread of the ion group in the z- and y- directions, the non-linear ion mirror being configured to reflect the ion group, at the turn-around point, along the ion flight axis towards the detector, the passage of the ion group through the non-linear ion mirror causing a spatial spread of the ion group in the x-direction at the detector due to the lateral spread of the ion group within the ion mirror, the time of flight mass analyser having at least one lens (34, 35, 44, 45, 54, 55) positioned between the ion source and the ion mirror, wherein the voltage provided to the or each lens is configured to reduce said lateral spread, so as to provide a local minimum of lateral spread within the ion mirror thereby minimising the spatial spread of the of the ion group in the x-direction at the detector; wherein the or each lens is positioned between the ion source and the ion mirror such that the distance from the mid-point of the ion source to the front edge of the lens is between 20% to 40% of the distance from the mid-point of the ion source to the back of the ion mirror. a pulsed ion source (30, 40, 50); a non-linear ion mirror (32, 42, 52) having a turn-around point; a detector (33, 43, 53); an ion flight axis extending from the pulsed ion source to the detector via the turn-around point of the non-linear ion mirror, the ion flight axis defining a x-direction; and a y-axis defining a y-direction and a z-axis defining a z-direction, the y-axis and the z-axis being mutually orthogonal and orthogonal to the ion flight axis, the pulsed ion source being configured to produce an ion pulse travelling along the ion flight axis, the ion pulse comprising an ion group, the ion group consisting of ions of a single m/z value, the ion group having a lateral spread in y- and z- directions, wherein said lateral spread is determined by an energy spread and a spatial spread of the ion group in the z- and y- directions, the non-linear ion mirror being configured to reflect the ion group, at the turn-around point, along the ion flight axis towards the detector, the passage of the ion group through the non-linear ion mirror causing a spatial spread of the ion group in the x-direction at the detector due to the lateral spread of the ion group within the ion mirror, the time of flight mass analyser having at least one lens (34, 35, 44, 45, 54, 55) positioned between the ion source and the ion mirror, wherein the voltage provided to the or each lens is configured to reduce said lateral spread, so as to provide a local minimum of lateral spread within the ion mirror thereby minimising the spatial spread of the of the ion group in the x-direction at the detector; wherein the or each lens is positioned between the ion source and the ion mirror such that the distance from the mid-point of the ion source to the front edge of the lens is between 20% to 40% of the distance from the mid-point of the ion source to the back of the ion mirror.