Correction of the second-order aberrations of uniform field magnetic sectors.

The focusing equations for uniform field magnetic sectors with circular edges are shown to have second-order double focusing solutions. The numerical methods used to find the appropriate magnetic sector parameters for moderate pole face separations are described. An electron spectrometer constructed for a field-emmission-gun-equipped Philips EM400 is shown to have an energy resolution of 0.5 eV at a sector half acceptance angle of 1 mrad.