The purpose of this paper is to help determine whether or
not a lightweight mirror is an appropriate engineering solution
in place of a more traditional, solid mirror. The relatively
recent manufacture of borosilicate lightweight mirrors both by
casting1 and by the Gas Fusion process' make this study timely.
Because lightweight borosilicate mirror substrates tend to. be
substantially less expensive than the equivalent lightweight
substrates made of very low expansion materials, a review of
the properties of lightweight mirror structures in general will
make it possible to determine if the cost savings of the
borosilicate substrates are justified when balanced against the
performance of the very low expansion mirror substrate materials.