Brittle materials such as glass do not possess a single characteristic strength. The strength of the material is dependent on the distribution of cracks or surface flaws. These factors, coupled with the inherent brittleness (cause of catastrophic or rapid failure) mean that extremely conservative design approaches are typically used for optical elements made of glass. Determining a design allowable for glass elements is critical for optical systems using relatively brittle glass types or for optical elements subject to relatively high stress levels. Rule-of-thumb tensile design strengths are typically at 1000 -1500 psi for nominal glass materials. This neglects the specific glass composition, subcritical crack growth, surface area under stress, and nature of the load -static or cyclic. Several methods to characterize the strength of optical glass are discussed to aid engineers in predicting a design strength for a given surface finish, glass type, and environment. These include estimating fracture toughness for a given glass, predicting inert strength using material test data, and lifetime predictions accounting for static fatigue and cyclic loading. Determining a design strength for a spaceborne optical element is discussed.

Automated design space exploration was implemented and demonstrated in the form of the multidisciplinary optimization of the design of a space-based telescope. Off-the-shelf software representing the industry standards for thermal, structural, and optical analysis were employed. The integrated thermal/structural/optical models were collected and tasked with finding an optimum design using yet another off-the-shelf program. Using this integrated tool, the minimum mass thermal/structural design was found that directly satisfied optical performance requirements without relying on derived requirements such as isothermality and mechanical stability. Overdesign was therefore avoided, and engineering productivity was greatly improved. This ambitious project was intended to be a pathfinder for integrated design activities. Therefore, difficulties and lessons learned are presented, along with recommendations for future investigations. * To better enable comparisons with the original design case, buckling was again neglected.

Productivity bottlenecks for integrated thermal, structural, and optical design activities were identified and systematically eliminated, making possible automated exchange of design information between different engineering specialties.The problems with prior approaches are summarized, then the implementation of the corresponding solutions is documented. Although the goal of this project was the automated evaluation of coupled thermal/optical/structural designs, significant process improvements were achieved for subset activities such as stand-alone thermal, thermal/ structural, and structural/optical design analysis. INTRODUCTION: PROBLEM STATEMENTStructural, thermal, and optical engineers typically work independently of each other using unrelated tools, models, and methods. Without the ability to rapidly exchange design data and predicted performance, and therefore to influence each other's efforts, the prior state-of-the-art for the design of advanced optical systems was inadequate: it has henceforth resisted attempts to achieve the ideals of concurrent engineering. Limited success has been achieved at a very top-level (suitable for conceptual design studies), but only by approximating or neglecting the detailed design tasks that the engineering specialist must perform in later mission phases.

The design, fabrication and testing of the BeamSplitter Assembly (BSA) of the Wide-field Infrared Survey Explorer (WISE) instrument are discussed in the paper. The BSA splits the WISE telescope optical output beam into 4 spectral wavelength bands: 2.8-3.8, 4.1-5.2, 7.5-16.5, and 20-26 µm. The BSA also provides focus adjustments to focus the WISE instrument prior to launch. The methods used to focus WISE are also discussed in this paper. Funding for and management of the WISE program were provided by the NASA Jet Propulsion Laboratory.

The Teal Ruby Experiment is an infrared telescope designed to passively operate in a cryogenic and orbital environment. As such, it had to be shown capable of maintaining integrity under a severe set of design criteria.Spacecraft payload capabilities required minimum optical element and structure weight while sufficient support strength was required to resist stresses developed under severe launch loading. Good stiffness characteristics were necessary to preclude excessive dynamic excursions as well as to minimize optical element motions caused by gravity release. Heat loss through supports which connect assemblies at different temperatures had to be kept to an extremely low value to assure life expectancy of the operational mission. Finally, low thermal expansion characteristics were a must if subassembly relative motions and cryogenic mirror distortions were to be held to the optical tolerances required.The recently completed telescope design satisfies these criteria and is presented in the paragraphs below. Here, a woven graphite epoxy composite structure houses lightweight fused silica mirrors. Variations in the coefficient of thermal expansion within the optical elements and structures are duly considered. The structural design and analysis, optical resolution capability, fabrication and manufacturing processes, and optical test results are discussed in detail.