A co-axial time-of-flight mass spectrometer comprising: first and second electrostatic ion mirrors (10, 11) arranged in opposed relationship on a common longitudinal axis; an ion source for supplying ions to a said ion mirror (11, 12) along an input trajectory, and ion detection means for receiving ions reflected at a said ion mirror (11, 12) along an output trajectory, after said received ions have performed at least one pass between said ion mirrors (11, 12); said mass spectrometer having:: said first and second ion mirrors (11, 12) are arranged repeatedly to reflect ions along said longitudinal axis, one of said ion mirrors is arranged selectively to reflect ions from said input trajectory onto said longitudinal axis and another of said ion mirrors is arranged selectively to reflect ions from said longitudinal axis onto said output trajectory said mass spectrometer being characterized in that: - said ions are supplied to a mirror along an input trajectory via a first isochronous point; - said ions are received at the detection means at or via a second isochronous point; - the input and output trajectory are offset from the longitudinal axis by an angle less than or equal to tan -1[D min /], wherein D min is the minimum outside transverse dimension of said ion mirrors, and L is the distance between the entrances of said ion mirrors; - at least one of said ion mirrors is arranged selectively to control a reflection angle whereby to enable ions to undergo multiple passes between the ion mirrors; - one of said electrode of each mirror is a tilting electrode which when selectively supplied, in use, with DC dipole voltage generates an electrostatic deflecting field effective to deflect ions relative to said longitudinal axis. first and second electrostatic ion mirrors (10, 11) arranged in opposed relationship on a common longitudinal axis; an ion source for supplying ions to a said ion mirror (11, 12) along an input trajectory, and ion detection means for receiving ions reflected at a said ion mirror (11, 12) along an output trajectory, after said received ions have performed at least one pass between said ion mirrors (11, 12); said mass spectrometer having:: said first and second ion mirrors (11, 12) are arranged repeatedly to reflect ions along said longitudinal axis, one of said ion mirrors is arranged selectively to reflect ions from said input trajectory onto said longitudinal axis and another of said ion mirrors is arranged selectively to reflect ions from said longitudinal axis onto said output trajectory said mass spectrometer being characterized in that: - said ions are supplied to a mirror along an input trajectory via a first isochronous point; - said ions are received at the detection means at or via a second isochronous point; - the input and output trajectory are offset from the longitudinal axis by an angle less than or equal to tan -1[D min /], wherein D min is the minimum outside transverse dimension of said ion mirrors, and L is the distance between the entrances of said ion mirrors; - at least one of said ion mirrors is arranged selectively to control a reflection angle whereby to enable ions to undergo multiple passes between the ion mirrors; - one of said electrode of each mirror is a tilting electrode which when selectively supplied, in use, with DC dipole voltage generates an electrostatic deflecting field effective to deflect ions relative to said longitudinal axis. - said ions are supplied to a mirror along an input trajectory via a first isochronous point; - said ions are received at the detection means at or via a second isochronous point; - the input and output trajectory are offset from the longitudinal axis by an angle less than or equal to tan -1[D min /], wherein D min is the minimum outside transverse dimension of said ion mirrors, and L is the distance between the entrances of said ion mirrors; - at least one of said ion mirrors is arranged selectively to control a reflection angle whereby to enable ions to undergo multiple passes between the ion mirrors; - one of said electrode of each mirror is a tilting electrode which when selectively supplied, in use, with DC dipole voltage generates an electrostatic deflecting field effective to deflect ions relative to said longitudinal axis.