As in the case of prisms, a clear understanding of key aspects of mirror design is necessary before we consider the various techniques available for mounting those mirrors. This chapter deals largely with the geometric configurations of different types of mirrors, their functions, and the reasons why they are designed as they are. Because mirror size is a prime driver of design and of material choice, we consider sizes ranging from small ones with diameters of a few millimeters to about 0.5 m (1.6 ft) to large, astronomical-telescope sized ones with diameters as large as about 8.4 m (27.6 ft). How the intended method of manufacture influences design is considered, as appropriate, throughout the chapter. We begin by listing the applications of mirrors, illustrating the uses of mirrors to control image orientation, and defining the relative advantages of first- and second-surface mirror types. How to approximate the minimum aperture dimensions for a tilted reflecting surface located in a collimated or noncollimated beam is considered next. We then describe various substrate configurations that might be employed to minimize mirror weight andâor self-weight deflection. Modern technology for thin facesheet adaptive mirrors is summarized briefly. Selected designs for metallic mirrors are considered. The chapter closes with a few observations about the design and use of pellicles.
8.1 General Considerations
Small mirrors usually have solid substrates shaped as right circular cylinders or rectangular parallelepipeds. They typically have flat, spherical, cylindrical, aspherical, or toroidal optical surfaces. Curved surfaces can be convex or concave. Usually the second, or back, surface of a small mirror is flat, but some are shaped to make the profile into a meniscus. The thickness of the substrate is traditionally chosen as 1â5th or 1â6th the largest face dimension. Thinner or thicker substrates are used as the application allows or demands. Nonmetallic substrates are typically borosilicate crown glass, fused silica, or one of the low-expansion materials (such as ULE or Zerodur). Metallic mirrors usually are made of aluminum unless some special requirement of the application leads to the choice of beryllium, copper, molybdenum, silicon, a composite material (such as graphite epoxy or silicon carbide), or a metal matrix material (such as SXA).
Most of the mirrors used in optical instruments are of the first-surface reflecting type and have thin-metallic-film coatings (such as aluminum, silver, or gold), which have protective dielectric coatings (typically magnesium fluoride or silicon monoxide). Second-surface mirrors have a reflective coating on the mirror's back; the first surface then acts as a refracting surface. The refracting surface typically carries an antireflection coating such as magnesium fluoride to reduce the effects of ghost images from that surface. A special mirror is the plate beamsplitter, which has a partially reflective coating on one surface to redirect some of the incident light and to transmit most of the rest.
Flat mirrors, used singly or as combinations of two or more, serve useful purposes in optical instruments, but do not contribute optical power and, hence, cannot form images by themselves.