Dimensional Stability of Materials Useful in Optical Engineering

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Abstract
Current requirements for increasingly stable structures have motivated the precision measurements described here. We survey the thermal expansivity versus temperature characteristics of some popular low-expansivity materials, including metals, for which the situation is especially unsatisfactory (for example, Invar and Super Invar) and point out the unusual expansion behaviour upon thermal cycling of Zerodur. A new differential technique is described which makes possible measurements of the uniformity of expansivity with an uncertainty as small as ±0·1 x 10-−10 K−1. Finally, we discuss ongoing measurements of temporal stability at constant temperature and call attention to the problems with Invars.