Interferometric texts were performed on a 1.8-m ULE lightweight mirror at different uniform temperature conditions to determine figure changes due to expansion coefficient inhomogeneity. The conclusions of the experiment were: (1) The rms value of surface figure changes due to material inhomogeneities were found to be 0.036λ over a 42°C soak range at 6328 Å. (2) Time variations of surface figure change were found to be insignificant over periods of up to 6 days. (3) No hysteresis remained after returning to figuring temperature (the measurement uncertainty was less than 0.023λ rms at λ = 6328 Å). (4) Analytical predictions of surface figure change based on α inhomogeneity data were in close agreement with measurements.
G. Friedman and G. Gasser, "Test Results on Homogeneity of Expansion for a 1.8-m ULE Lightweight Mirror," Appl. Opt. 11, 2768-2777 (1972)