Limits of Resolution in Magnetically Focused Image Converter Tubes

In a simple image tube in which the uniformly accelerated electrons are focused by a homogeneous magnetic field onto the phosphor, the error in focusing due to the aribtrary initial velocity of the photoelectrons is calculated. It is shown that optimm resolution can be obtained if focusing is adjusted for electrons leaving the cathode at the average initial forward velocity. A method to measure the initial average transverse velocity in the tube itself is developed. A practical formula for the expected resolution is derived (see Equation 12) for tubes with S-11 photocathodes and for the incident light coming from a slightly underheated incandescent lamp.