Tunable ultra-stable composites and laser interference dilatometry

Building an ultra-stable composite with a sub-ppb/K coefficient of thermal expansion uncertainty for the Habitable Worlds Observatory requires precise control over the composite manufacturing process and metrology. The ultra-stable composite architecture and manufacturing process needs to be developed, then its coefficient of thermal expansion measured to subppb/K uncertainty. This paper evaluates tunable ultra-stable composites to enable post-manufacturing coefficient of thermal expansion adjustment and a dual-sided laser interferometric dilatometer to measure the coefficient of thermal expansion. A tunable ultra-stable composite with fused silica and sapphire is being developed, offering ±250 ppb/K coefficient of thermal expansion tunability. A dual-sided laser interferometric dilatometer was constructed, and preliminary tests were performed by measuring a steel sample with mirrors attached. The fused silica and sapphire ultrastable composite has been a good platform for developing tuning methods, but it needs to be adapted to carbon fiber reinforced polymer (the likely choice for the Habitable Worlds Observatory). The use of a dual-sided laser interferometric dilatometer is not as beneficial when compared to measuring other naturally reflective low coefficient of thermal expansion materials, but it might still provide some benefit over its single-sided counterpart.