Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Modified Dallâ€“Kirkham telescope utilizes an elliptical primary and spherical secondary mirror as in the conventional Dallâ€“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 Dallâ€“Kirkham and must be optimized along with the lenses during design. The usable field is much better than the Ritcheyâ€“ Chretien telescope without corrector, and over very wide spectral bands, typically 0.38 to 0.95 micrometres if the corrector is made of quartz. The Ritchey Chretien System with corrector has a better and bigger field.