Abstract: A rigid thermally stable structure provides a platform on which devices may be supported in precise positional relationships. The platform is provided by a thin plate of material having a coefficient of thermal expansion which is relatively low, such as may be fabricated from an iron nickel alloy of the type manufactured under the trade names Invar and Super-Invar. A rigid base of material which may have a high thermal expansion coefficient is also provided. The thin plate is attached to the rigid base by an array of posts which imparts thereto the rigidity of the base for preventing flexural movement of the plate while permitting the base and plate to move with respect to each other as their longitudinal dimensions change with temperature. The thin plate therefore provides a rigid and thermally stable platform.