Registration marks for electron lithography are usually detected by scanning with an electron beam and measuring the backscattered electron (BE) signal. If the registration mark is on the axis of the lithography tool, the BE signal is symmetric and the mark position may be found accurately. Practical electron lithography often requires that registration marks be found off axis, for example, near the edges of an exposure field. In this case, it is shown by both experiment and Monte Carlo calculation that the registration signals become asymmetric which causes registration errors. It is also shown that these errors can be reduced by a suitable choice of BE detector shape and position.