IMAGING OF EXTRAAXIAL POINTS BY ZONE PLAT ES - STIGMATISM AT A PARTICULAR ORDER

We present exact analytical expressions for the boundaries of individu al zones of a zone plate that yields stigmatic imaging between two ext raaxial conjugate points. Our treatment incorporates imaging at any or der. It is found that the boundaries of the zones are elliptical and t hey are not concentric. We introduce dimensionless variables to arrive at convenient expressions for eccentricity and shift to the centre of the elliptical zone boundaries. Our approach enables us to obtain sim ple formulae for the two parameters. Approximations involved in the sa me are valid for most practical applications.