ABSTRACT
Our concern is with axisymmetric lenses and so we use cylindrical polar coordinates with the z-axis as the axis of symmetry of the lens system. Laplace’s equation is
∂2V
∂z2 + 1 r
∂
∂r
( r ∂V
∂r
) = 0 (2.1)
where V (r, z) is the potential. Because of the rotational symmetry an expansion of the potential in even powers of r can be made
V (r, z) = ∞∑ n=0
An(z)r 2n.
