High-frequency asymptotics of MTF in image tubes

High-frequency properties of the modulation transfer function (MTF) in electrostatic image tubes with axial symmetry are studied in detail. Based on aberration theory and stationary phase technique, simple but quite precise expressions for MTF asymptotics in the high-frequency range with regard to arbitrary photoelectrons energy and angular distributions are obtained. Some important partial cases are considered, and a comparison between the asymptotic and numerical results is carried out.