One of the key parameters usually considered in a light weighted mirror design is its flexural rigidity. In part I of this paper, the relative merits of symmetric sandwich and open back light-weighted designs in the 10 to 30 kg/m2 range (of mirror weight per unit area) are examined for ULE, Heraeus fused silica, Zerodur and beryllium, subject to some representative state-of-the-art fabrication constraints. Part II of the paper addresses the same issue independently of any specific material. This is made possible by normalization of several parameters by the weight equivalent solid thickness of the mirror. Also included in the study are nonsymmetric sandwich cross sections with unequal front and rear facesheet thicknesses. The study shows that, for given values of mirror weight per unit area and the core (or rib) solidity ratio, there exists a transitional value of the normalized front facesheet thickness above which an open back configuration is structurally superior to a sandwich configuration. Moreover, for the increasingly demanding light-weighting goals of moderns optical systems, the optimum geometrical configurations may be beyond the fabrication capability of the current state of the art.
Pravin K. Mehta,
"Flexural Rigidity Characteristics Of Light-Weighted Mirrors", Proc. SPIE 0748, Structural Mechanics of Optical Systems II, (3 June 1987); doi: 10.1117/12.939828; https://doi.org/10.1117/12.939828