Abstract: A concave mirror construction and a method of making concave mirrors, particularly of large focal length. A normally flat rigid glass mirror is held in a concave configuration under bending stress within its elastic limit by a holding layer of substantially dimensionally stable material bonded thereto. The holding layer is formed by a layer of open-ended expansible cellular material sandwiched between layers of pliable and solidifiable dimensionally stable material. The holding layer is bonded to the glass mirror while the mirror is being mechanically held in a concave configuration under bending stress within its elastic limit. The holding layer is cured in situ to a solid and dimensionally stable state which holds the glass mirror in the concave configuration.