A compact and robust focus drive mechanism was developed for the International Ultra-violet Explorer (IUE) scientific instrument. The design philosophies and detail design features of this mechanism are discussed. The mechanism design is based on a novel planetary-type drive which converts angular increments of a permanent magnet stepping motor into simultaneous linear displacements of three ball screws which support the secondary mirror. The design produces significant torque margins while providing an exceptionally smooth and reliable operation. Materials selections, driven by bearing and ball screw requirements, are also discussed. A summary of performance capabilities is included.