Variable magnification image tube

Claim 1

In a variable magnification image intensifier tube, means forming a photocathode for converting a photon input image incident upon said photocathode into an electron image pattern emitted from said photocathode, an image converter screen for converting the electron image pattern into an intensified output optical image, means forming an electrode structure disposed intermediate said photocathode and said image converter screen for accelerating and focusing the emitted electron image pattern onto said image converter screen, said electrode structure including an anode electrode means, a zoom electrode means, and a focus electrode means, said zoom electrode means being disposed between said photocathode and said anode electrode for varying the magnification of the electron image incident on said image converter screen over a certain range of magnification having a high end and a low end in variable accordance with a potential applied to said zoom electrode means, said zoom electrode having an electron entrance opening and an electron exit opening through which electron images pass to said converter screen, said focus electrode means being disposed imtermediate said zoom electrode and said photocathode for focusing the electron image upon said image converter screen, the improvement WHEREIN, the size of said electron entrance opening is dimensioned such that the electron image pattern incident on said image converter screen is in focus at both the high and low ends of said magnification range for substantially the same magnitude of focus potential applied to said focus electrode means.