The mirror elements are made of Zerodur, a glass ceramic made by Schott in Germany. The wall thicknesses are about 23.6, 18.3, 16.5, and 15.9 mm for pairs 1, 3, 4, and 6, respectively, providing similar stiffnesses against oval distortions during fabrication and metrology. These thin walls would result in unacceptable mirror distortions as a result of self-weight induced deflections if the mirror elements were not supported carefully. The flight mirror elements are supported at their centers with graphite composite sleeves; this arrangement reduces the performance destructive mode in which the mirror ends become oval with the major axis at the small end being perpendicular to the major axis at the large end. The composite sleeves are supported by an aluminum center aperture plate. The Zerodur and composite materials were selected in part for their low coefficients of thermal expansion (the composite sleeves isolate the mirror elements from the effects of the aluminum center aperture plate), which allows achievable tolerances on temperatures during metrology, alignment, and operation.