Found this in a Rauland patent for a electrostatic focus CRT:
Quote:
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it has been found essential that the electron beam be centrally directed along the axis in its passage through the focusing system. In other words, the ion trap must be precisely adjusted for satisfactory focusing with a low-potential electrostatic lens system. Such precise alignment of the ion trap is facilitated by providing a fluorescent coating 29 on the outer wall of second accelerating electrode I6. This fluorescent coating serves as an ion-trap adjustment indicator; in practice, permanent magnet 21 is -moved both axially and rotationally until the glow from fluorescent coating 29 is reduced to a minimum, thus indicating precise ion trap alignment.
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https://www.google.com/patents/US2627047
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