The Modified Dall-Kirkham

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

utilizes an

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primary and

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

secondary

mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

as in the conventional Dall-Kirkham configuration, but also includes a lens group (usually two or three

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

elements) ahead of the focal point to improve off-axis

image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...

quality. The primary mirror conic constant is slightly different from that for a conventional Dall-Kirkham and must be optimized along with the lenses during design. The usable field is much better than the Ritchey-Chrétien telescope without corrector, and over very wide spectral bands, typically 380 to 950

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s (edges of

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and

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) if the corrector is made of quartz. (With a corrector, the Ritchey-Chrétien System also has a better and bigger field.) Such a telescope was designed by Rosin and Wynne after

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. The performances are equal or better than the Ritchey-Chrétien telescope. The spherical secondary can be fringe tested against a spherical concave surface or tested from behind. This is markedly an advantage over the hyperbolic secondary of the Ritchey-Chrétien design. Another advantage of either the basic Dall-Kirkham or the Modified Dall-Kirkham design is that collimation of the convex spherical secondary mirror with respect to the optical axis of the primary mirror is almost trivial, because there is no single defined axis of a sphere. Any line that runs through the center of the sphere can be an axis.

References

Le télescope de Wynne-Rosin
(French)

References

See also