[en] Large, optical-quality, metallic reflectors up to 2.0 m in dia are required for use in the High Energy Gas Laser Facility (HEGLF) at the Los Alamos Scientific Laboratory. Mirrors of this quality and size can only be made by diamond turning on a highly specialized Ex-Cell-O machine. The 975-kg weight of aluminum stock for a 2-m mirror greatly exceeds the allowable 545-kg total weight of part and chuck on the Ex-Cell-O machine. Accordingly, a program was initiated to develop highly rigid, lightweight mirror substrates for this application. A unique core material (Tubecomb) was developed for economically fabricating these mirrors from commercially available materials. Such mirrors will weigh only 15 to 25% as much as solid metal and will be more rigid. A 267-mm-diameter prototypic Tubecomb mirror was made. Two 406-mm test mirrors have been designed and one has been built for determining the long-term stress-strain-creep behavior of such structures. Tooling and fixturing have been designed for optically evaluating the short and long-term mechanical properties of the Tubecomb mirrors