Parabola-doublet aplanat becomes anastigmatic when second doublet is inserted near focus

A doublet of choice glasses may be located in the converging focal cone of the infinity-focused parabola to yield an aplanatic telescope or camera. The resulting angular field is limited by high astigmatism but is significantly larger than that of the coma-limited parabola. The spherical and chromatic aberrations are so well corrected and the coma so well balanced that the doublet may be used unaltered with a parabola of arbitrary focal length and speed with excellent results for the unvignetted rays. A second doublet nearer to the focus and designed independently of the first corrects the system's astigmatism while preserving its aplanaticism. It may also be designed for flattening the field. This arrangement may allow for greater flexibility in the placing of optical elements than does Wynne's triplet for modest-aperture systems. Equations are presented for choosing candidate glasses for the first doublet from the very limited manifold of solving glasses.