A magnetic lens system designed from studies made with computing machines has been built and used at the Brookhaven cosmotron. It collected pions of 1020 MeV/c mean momentum produced at a target in the external proton beam, selected a narrow momentum bandwidth from a continuous momentum spectrum, recombined the selected momenta, and refocused the pions to an image of unit magnification. The effective solid angle of acceptance was 5.04×10−3sr and the momentum band pass width was ±30 MeV/c. The measured particle yield on 1 in.2at the focus was 5×105for 6×1010internal 3BeV/c protons; 27% were positive pions and 73% were protons.