Optics Program Simplifies Analysis and Design
- Monday, 14 April 2008
Future spaceborne astronomy missions will require telescopes with increasingly greater power, driving the dimensions of the optics and their housing structures to significantly greater sizes.
The increased size of the structures reduces the dynamic frequencies of the optical system, to the point where disturbance frequencies and structural modes significantly interact. At the same time, the requirements on dynamic stability to achieve the required optical performance are significantly tighter than for anything that has flown before, and, therefore, the sensitivity to dynamic effects is correspondingly high. Finally, the physical size of the optical instruments makes fully integrated system-level testing extremely problematic. Not only are the systems too large to test in any existing environmental chambers, they are susceptible to gravity loading effects and suspension coupling that will significantly change the dynamics. Validation of the large designs must then rest on a combination of analysis and system tests.
Techniques for system tests include using Finite Elements (FE) and FE model updating tools, system identification, and using various other tools for performing optical analyses. These techniques, however, do not provide for analysis of cross-disciplinary results. Therefore, the conventional approach is to develop a requirement budget that assigns error allocations to each of the modeling teams. This approach is extremely limiting in that it does not allow requirements to be freely traded among subsystems. NASA, intent on sending larger, more powerful optics into space, resolved to find a better way to test them.
Engineers at Goddard Space Flight Center partnered with software experts at Midé Technology Corporation, of Medford, Massachusetts, through a Small Business Innovation Research (SBIR) contract to design a new analysis system.
The result of the two-phase contract was the Disturbance-Optics- Controls-Structures (DOCS) Toolbox, a software suite for performing integrated modeling for multidisciplinary analysis and design. The Toolbox allows the definition of subsystem/component models, including structural models, control system models, optical sensitivities, and disturbance models. The component models are automatically coupled together to create a mathematic model of a complete physical process, using techniques that maximize the numerical conditioning, while maintaining modeling accuracy.
The code has been validated and applied to the following NASA astronomy projects and facilities: the Terrestrial Planet Finder Structurally Connected Interferometer Testbed (TPF-SCIT), the Terrestrial Planet Finder Coronagraph (TPF-C), the James Webb Space Telescope, and the Solar Dynamics Observatory.
The purpose of the DOCS Toolbox is to integrate various discipline models into a coupled process math model that can then predict system performance as a function of subsystem design parameters. The Toolbox accepts as input the discipline models from a variety of currently available discipline modeling tools. The Toolbox then connects the discipline models and applies numerical conditioning algorithms to improve the numerical accuracy, while still maintaining model accuracy. It performs the analysis and redesign in a graphical framework that allows the user to define and solve the analysis problem, and it then documents results in a point-and-click environment.