A Mersenne-Cassegrain telescope provided in a single block of glass in which opposed parabolic elements are precision milled through diamond turning of a glass boule, with the magnification power of the telescope determined by the differences in focal length between the two parabolas. The result is a volumetrically small telescope with pre-aligned surfaces that are maintained by the structural rigidity of the glass itself and in which thermal coefficients of expansion, vibration and the like have no effect due to the single glass element structure.