Design of an objective lens with a minimum chromatic aberration coefficient

Limit of the axial chromatic aberration coefficient Cc at 200 kV is numerically calculated. The obtained minimum is Cc=0.63 mm and the corresponding spherical aberration coefficient Cs=0.39 mm for the objective lens with the gap length 1.2 mm and the bore diameter 0.4 mm. The estimated resolution of the information limit is 0.07 nm under the high voltage stability 1×10⁻⁷ and the field emission gun. We consider a Wien filter monochrometer inside the gun to further improve the resolution.