Abstract: A time of flight analyser that comprises a pulsed ion source; a non-linear ion mirror having a turn-around point; and a detector. The pulsed ion source is configured to produce an ion pulse travelling along an ion flight axis, the ion pulse comprising an ion group consisting of ions of a single m/z value, the ion group having a lateral spread. The non-linear ion mirror is 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. The time of flight mass analyser has at least one lens positioned between the ion source and the ion mirror, wherein 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.