Evaluation of Dimensional Stability of Metering Truss Structure Using Built-in Laser Interferometric Dilatometer

Dimensional stability in a space telescope is one of the important factors for performing high-resolution observations. The supporting structures between the primary and the secondary mirrors in a space telescope are required to maintain the mirror positions with the proper focus for minimizing the optical alignment error. In orbit, the harsh environment subjects a space telescope to temperature variations that can lead to deformation of those structures and degradation of the telescope's optical performance. Several approaches are available for achieving high dimensional stability. They include minimizing temperature variations with active thermal control, designing the structure with low thermal expansion materials, and correcting the shape and/or position of the structure with actuators. An optimum combination of these measures is determined based on an accurate evaluation of the dimensional stability of the structures. This paper proposes a displacement measuring interferometer system with a simple, robust, and compact sensor that can monitor the dimensional stability of the precise structure by integrating the sensor into the structure. In this technique, a Fabry-Perot displacement sensor was built into the end of a strut that is a component of a metering truss structure. Two types of truss struts were tested to verify the performance of the proposed technique in measuring thermal expansions. The first one was the prototype that was made of stainless steel (SUS304). The other strut was made of a low thermal expansion ceramic (SiAlON) that is one of the most promising materials for a highly thermo-stable satellite structure. The thermal dimensional stability of these struts was evaluated by using the proposed technique and compared with conventional dilatometers for validating this new technique. The results showed that the technique has similar precision to the conventional measurement system and provide a more convenient and stable measurement system.