A co-axial time-of-flight mass spectrometer having a longitudinal axis and first and second ion mirrors at opposite ends of the longitudinal axis. Ions enter the spectrometer along an input trajectory offset from the longitudinal axis and after one or more passes between the mirrors ions leave along an output trajectory offset from the longitudinal axis for detection by an ion detector. The input and output trajectories are offset from the longitudinal axis by an angle no greater than formula (I): where Dmin is the or the minimum transverse dimension of the ion mirror and L is the distance between the entrances of the ion mirrors.