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Chapter 15:
Two-Mirror Systems as Telescope and Microscope Objectives
Abstract
15.1 Introduction In 1663, the Scottish mathematician and astronomer James Gregory suggested a two-mirror system with a parabolic primary and an elliptical secondary mirror. The configuration is shown in Fig. 15.1. At that time, he was unable to find an optician to grind and polish the elliptical mirror. Nine years later, in 1672, the Frenchman Guillaume Cassegrain came up with the idea to intercept the reflected rays from the primary mirror before they come to a focus. For that purpose, a hyperbolic secondary mirror is needed. Cassegrain's system is much shorter and has found its place in many applications—from astronomical telescopes to thermal imaging devices. It is interesting to note that Isaac Newton, the great British scientist, never gave Cassegrain any credit for his invention. He referred to it as “just an obvious extension” of Gregory's concept. If the field of view is narrow, only spherical aberration has to be corrected. Dutchmen Horace Dall and Allan Kirkham conceived of the idea to replace the hyperbolic secondary mirror with an easierto-manufacture convex spherical mirror. To keep the system free from spherical aberration, the primary mirror becomes, in that case, an ellipsoid. This increases coma somewhat, but that was not of much concern to the two astronomers. In the 1920s, the American George Ritchey and the Frenchman Henri Chretien modified the basic Cassegrain arrangement so that it is corrected for spherical aberration and third-order coma. This Ritchey-Chretien configuration is now the most widely used combination.
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CHAPTER 15


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