Test Results on Homogeneity of Expansion for a 18-m ULE Lightweight Mirror

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 0.036lambda over a 42 degrees C soak range at 6328 A. (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 0.023lambda rms at lambda = 6328 A). (4) Analytical predictions of surface figure change based on alpha inhomogeneity data were in close agreement with measurements.