A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better thanm/&Dgr;m=500full width at half maximum and 250 full width at 10&percent; maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys. ©1998 American Institute of Physics.