27.1 INTRODUCTION

The Schwarzschild objective is based on the use of two almost concentric spherical surfaces: a small convex mirror and a larger concave facing each other as shown in Fig. 1.

While this design is widely attributed to Schwarzschild, previous descriptions of the optical system had already been analyzed and published by Paul and Chretien. The design is simple and elegant, and well suited for optical systems with small field of view and high resolution. The Schwarzschild objective is an evolution of the Cassegrain telescope, where the primary and secondary are both nonspherical elements, providing good aberration correction and large field of view. Aspherical optics are, however, difficult and expensive to manufacture, and simpler designs are desirable. The Schwarzschild objective replaces the aspheres with spherical elements, as shown in Fig. 1.

The Schwarzschild objective has found its primary use in microscopy and astronomy at wavelengths where glass lenses are not suitable or where a truly achromatic optical is needed, like in microspectroscopy systems. The applications are thus mainly in the Infrared (FTIR microscopes) and the UV; recently, the Schwarzschild objective has been extended to the Extreme UV region ( ?13 nm) for microscopy ^{[?????] ,}4 7 and for the development of advanced lithography.3 Some of the first X-ray images of the sky were also acquired using Schwarzschild objectives.4

In the Schwarzschild objective each spherical surface forms an aberrated image, and it is a...