Field Flatteners

As we've discussed, all standard two-mirror telescopes have curved focal planes. It is possible to make a simple lens to correct the field curvature. One can derive that if a lens is placed in (or near) a focal plane, it does not contribute any significant aberrations (though if one were really working in a diffraction-limited system, one would have to check this carefully). Consequently, we can put a lens right near the telescope focal plane to flatten the field. For a field which curves towards the secondary mirror, one finds that the correct shape to flatten the field is just a plano-concave lens with the curved side towards the secondary.