Modified Dall–Kirkham telescope

Lore & Background

The Modified Dall–Kirkham telescope was designed by Rosin and Wynne after World War II. It builds upon the basic Dall-Kirkham design by incorporating a lens group ahead of the focal point, which improves off-axis image quality. The primary mirror conic constant is slightly different from that of a conventional Dall-Kirkham and must be optimized along with the lenses during design.

The spherical secondary mirror offers practical advantages: it can be fringe tested against a spherical concave surface or tested from behind, which is markedly easier than testing the hyperbolic secondary of the Ritchey-Chrétien design. Additionally, collimation of the convex spherical secondary mirror with respect to the optical axis of the primary mirror is almost trivial, because any line through the center of a sphere can serve as an axis.

Reader's Guide

The Modified Dall–Kirkham telescope achieves performance equal to or better than that of the Ritchey-Chrétien telescope, according to the source article. Its usable field is much better than that of the Ritchey-Chrétien without corrector, especially over very wide spectral bands (380 to 950 nanometres) when the corrector is made of quartz. However, the article notes that with a corrector, the Ritchey-Chrétien system also has a better and bigger field. The design's spherical secondary mirror provides significant practical benefits: it is easier to test than a hyperbolic mirror, and collimation is simplified because a sphere has no single defined axis. These characteristics make the Modified Dall–Kirkham a notable alternative to the Ritchey-Chrétien, particularly for applications requiring wide-field performance and simpler optical fabrication and alignment.

Did You Know?

• The Modified Dall–Kirkham uses an elliptical primary and spherical secondary mirror, like the conventional Dall-Kirkham.
• Its lens group typically consists of two or three elements placed ahead of the focal point.
• Collimation of the spherical secondary is almost trivial because any line through the sphere's center can be an axis.