In Situ Measurement of Environmentally-Induced Deformations of Telescopes

Two metrology techniques, suitable for measuring small (micron-scale) deformations of relatively large in situ telescope structural support members, have been developed for determining the coefficients of thermal and moisture expansion (CTE and CME) of carbon fiber-epoxy (CFE) composite telescope components. Both are relative (rather than absolute) techniques, measuring the axial deformation of the telescope component with respect to a known reference standard. Two techniques are described: a single mirror optical lever and a double mirror optical lever. In the first method, the temperature of the test component is varied while the reference standard is maintained at constant temperature, while in the second method the temperatures of the test component and the reference standard are varied together by changing the ambient temperature surrounding both. Both techniques are demonstrated in this paper; the second technique is shown to be a significant improvement upon the first and also a significant improvement upon comparable techniques reported in the literature. Quantitative CTE and CME results are reported for IM7 pultruded CFE rods.