Concentric and confocal systems for adaptation as spectrographs

Two sufficient conditions for good unit-magnification imagery are that the optical system be concentric and retroreflecting. These conditions are satisfied by both the Dyson1 and the Offner2 systems. Schmidt corrector plates as shown in figure 1 are seen to extend the field as we note that the Offner system is a Schmidt telescope with image and pupil planes interchanged. Moving the corrector’s neutral zone outward leads to whispering gallery systems as shown in figure 2, where again the Schmidt plates can widen the annular field. In analogy with the successful inclusion of a diffraction grating in the Dyson system3, it is speculated that holographic gratings could also be used on the mirrors marked G in the systems of figure 2 to obtain stigmatic flat field spectrogaphs. Figure 3 shows a folded version of figure 2A. Figure 4 shows how to evolve an ellipsoidal confocal aplanatic system to a Czerny-Turner system along the style of an Offner spectrograph.