Time-of-flight mass spectrometers on spacecraft are the most direct method for determining chemical composition of cosmic dust grains. Miniaturization of these instruments presents many challenges. Larger space-charge effects, greater deviations from the paraxial approximation, and various ion-optical aberrations negatively affect mass resolution in small time-of-flight instruments. We report on the building and testing of an instrument design that may reduce these effects. In addition to a linear reflectron, ions pass through a ring aperture that transmits only those ions with transverse velocity components that fall within a specific range. This novel design focuses ions onto a detector with greatly reduced spherical aberration. Space-charge effects and the effects of impact plate cratering and grid scatter are also reduced using this design. Controlled impacts of iron microparticles at several km/s demonstrate instrument performance. This instrument is suited for characterization of cosmic dust in regions of very high dust flux, such as a comet flyby, and it may also have practical laboratory or field applications.