Modified Dall-Kirkham telescope
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The Modified Dall-Kirkham telescope utilizes an elliptical primary and spherical secondary mirror as in the conventional Dall-Kirkham configuration, but also includes two lens elements ahead of the focal point to improve off-axis image quality. The primary mirror conic constant is slightly different than that for a conventional Dall-Kirkham and must be optimized along with the lenses during design. Usable fields can be three to five times larger than the conventional Dall-Kirkham field, and over very wide spectral bands, typically 0.38 to 0.95 micrometres.
Such a telescope was designed by Rosin and Wynne after WWII. The performances are equal or better than the Ritchey-Chretien telescope. The spherical secondary can be fringe tested against the spherical calliper. This is markedly an advantage over the hyperbolic secondary of the RC. There are some observatories using such optical combination.
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