Fig. (11) An iron-free magnetic β-ray spectrometer with radial focusing and axial defocusing at the curved exit slit [108]. This focusing principle offers better combination of resolution and transmission than conventional π √2 spectrometers at the price of a larger detector and focusing angle of (π/2)·√13 = 324.5°. The radius of a central orbit is 30 cm. Note the coils compensating external magnetic fields in three perpendicular directions.