Thermal deflection tests were run on two ULE fused silica mirror blanks, one solid and one of eggcrate construction, loaned to Itek Corporation by Corning Glass Works. The tests were accomplished by radiant heating of one surface while the deflection of the other surface was observed by means of holographic interferometry. Primary test objectives were (1) the mea-surement of an average thermal expansion coefficient for each blank, and (2) the detection of any unsymmetrical deflection. The thermal expansion coefficient values obtained were 1.1 x 10-7 per °C for the solid blank (30 inches in optical diameter by 4.15 inches thick) and 0.23 x 10-7 per °C for the eggcrate (see Fig. 1). No significant deviations from a uniform spherical deflection were seen. We believe that the error in these measurements does not exceed 0.15 x 10-7 per °C.