TABLE OF CONTENTS The increasing use of optical systems in the nonvisual regions of the spectrum, i.e., the ultraviolet and infrared regions, has resulted in a corresponding increase in the use of reflecting optics. This is due primarily to the difficulty in procuring completely satisfactory refractive materials for these regions, and secondarily, to the fact that many of the applications permit the use of relatively unsophisticated mirror systems.
The material difficulty is of two kinds. Many applications require the use of a broad spectral band, and a refractive material must transmit well over the full band to be of value. Secondly, chromatic aberration can be difficult to correct over a wide spectral band, and the residual secondary spectrum is sometimes intolerable. A review of Chap. 10 will demonstrate quite clearly the advantages of a reflector in this regard; an ordinary aluminized mirror actually has much better reflectance in the infrared than in the visible and (with special attention) aluminum mirrors suitable for the ultraviolet can be fabricated. Pure reflecting systems are, of course, completely free of chromatic aberration over any desired bandwidth.
The simplest reflecting objective is the spherical mirror. For distant objects the spherical mirror has undercorrected spherical aberration, but the aberration is only one-eighth of that of an equivalent glass lens at "minimum bending." The sphere is an especially interesting system when the aperture stop is located at the center of curvature, as shown in Fig. 18.1, because the system is then monocentric, and...
