  A new method to reconstruct charged fragment four-momentum vectors from
measured trajectories behind an open, large gap, magnetic dispersion element (a
sweeper magnet) has been developed. In addition to the position and angle
behind the magnet it includes the position measurement in the dispersive
direction at the target. The method improves the energy and angle resolution of
the reconstruction significantly for experiments with fast rare isotopes, where
the beam size at the target position is large.
