The Measurement of Yoke Astigmatism

Astigmatism, the principal deflection yoke aberration, sets a fundamental limit on the performance of cathode ray tube displays and is possessed by all yokes in varying type and degree. This paper analyzes the structure of an astigmatic beam with a view to measuring its properties objectively. Optical concepts are useful in describing the beam distortions encountered and two methods of achieving a numerical rating are outlined. Microscope measurements of the variation of spot shape along the beam path yield quantitative data regarding the primary and secondary foci. This data provides a basis for isolating the ``optical´´ characteristics of deflection yokes. Methods of classifying and evaluating astigmatic errors in both monochrome and color displays are discussed.