Techniques for Mounting Metallic Mirrors

• Published in: Mounting Optics in Optical Instruments p. 1
• Main Author: Yoder Paul R. Jr
• Format: Book Chapter
• Language: English
• Published: SPIE 2008; SPIE Press
• Edition: 2nd Edition
• Subjects: Geometric & Physical Optics; Optics & Photonics
• ISBN: 9780819471291; 0819471291
• DOI: 10.1117/3.785236.ch10

The design of metallic mirrors was discussed in Section 8.8 of this work. Because the design of the mirror itself and the design of its mounting are closely interrelated in many cases, examples of a few mountings were also described in that section. For example, Figs. 8.41 showed how the aluminum secondary mirror for the Kuiper Airborne Observatory was attached with screws to the hub of a tip∕tilt drive mechanism. Providing a proper interface between the mirror and the hub and the requirement for minimal weight along with high stiffness so the mechanism could move the mirror at high acceleration rates were key features of the mirror design. Similarly, the mountings for the beryllium primary segments of the James Webb Space Telescope, described in conjunction with Fig. 8.45, are closely tied to the design of the mirrors. In this chapter, we delve more deeply into the specifics of mounting metallic mirrors. First, we consider how metal mirrors and their interfaces with the mountings can be shaped precisely by single point diamond turning. Then, we consider integral mountings wherein features of the mirror substrate are configured to attach directly to the mechanical support. Provision of flexures in the mounting features of larger metallic mirrors is then described. These flexures serve to minimize optical surface distortion effects due to mounting forces. We consider how platings applied to the mirror surfaces to provide suitable material for polishing or diamond turning affect the optical behavior of the mirror when the temperature changes. Heat transfer through the mechanical interface plays a key role in such cases. Finally, we describe how metallic mirrors and their mounts can be configured to facilitate assembly and alignment. Single crystal diamond cutting tools and specialized machinery are used to create flat or curved surfaces on a variety of materials by very accurately cutting away thin layers of the surface. This process is variously called “single point diamond turning,” “precision machining,” or “precision diamond turning.” We here adopt the first terminology and abbreviate it as SPDT. The process has developed from crude experiments to fully qualified production processes since the early 1960s. See, for example, Saito and Simmons, Saito, Sanger, and Rhorer and Evans. The SPDT process generally involves the following steps: (1) preform or conventionally machine the part to rough shape with approximately 0.1 mm (0.004 in.) excess material left on all surfaces that will be processed, (2) heat treat the part to relieve stress, (3) mount the part with minimal induced stress in an appropriate chuck or fixture on the SPTD machine, (4) select, mount, and align the diamond tool on the machine, (5) finish machine the part to final shape and surface quality with multiple light cuts under computer control, (6) inspect the part (in situ, if possible), and (7) clean the part to remove cutting oils and∕or solvents. For some applications, plating the surfaces following step (2) is required to provide an amorphous layer of material to be diamond turned. Sometimes step (7) is followed by polishing the optical surface or surfaces to smooth it or them. An appropriate optical coating may then be applied if required for the application.