Dimensional instability of materials: how critical is it in the design of optical instruments?

Dimensional instability exists to some extent in all components no matter what the materials may be. So the question is not "how can we eliminate instability?" but rather, "how can we reduce it to a tolerable level?" The maximum allowable dimensional instability will vary with application and depends on the particular component and its role in the optical instrument. The purpose of this paper is to provide the basis for deciding how much can be tolerated and for making intelligent choices in the selection of materials and processes for components that will achieve stability design goals with which to meet optical instrument performance specifications. This basis is a better understanding of the causes of instability and methods for minimizing instability. After a discussion of tolerable levels of instability, four types of dimensional instability are defined: thermal, temporal, cycling and hysteresis, with examples given for each. The principal causes of these instabilities: external stress, changes in internal stress, microstructural changes and inhomogeneity/anisotropy of properties, are explained in some detail along with a discussion of material types and properties. Most importantly, methods for minimizing the instabilities are shown. This discussion includes specific recommendations for commonly used materials including: processing techniques to minimize instability, specific problems observed in some materials and how to avoid the problems, and some general guidelines regarding the effects of fabrication methods on stability. It is most important to realize that increasingly tighter specifications for optical instruments mean that the optomechanical designer must work concurrently with other engineering disciplines, particularly materials and processes engineers, to insure the desired thermal and temporal stability of the product.