A field‐plotting and ray‐tracing technique based on electron‐optical methods is developed for first‐order ion optics of slit lenses and is applied to a Nier‐type thick lens. A resistance paper analog gives the distribution of electrostatic potential. Maxwellian velocities of ions emitted from the filament and lens thickness affect ion trajectories, which are calculated to a few percent accuracy by a simple approximation technique. Paraxial ray paths are shown and the ion‐focusing action of the single filament is described. Optimum detectability, 100 counts of the smallest isotope, is 5×10−15g for uranium and 2×10−15g for plutonium. Ionization at moderate temperatures improves transmission and avoids background impurities which appear at temperatures above 2000°C. Independent electrostatic focusing in thezdirection improves transmission through the analyzer.