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Large ground-based observatories and future space-based astronomical observatories will rely increasingly on optical systems containing active image maintenance. A near-term example of a space-based system that will rely on this technique for ensuring adequate imaging performance is the Next Generation Space Telescope. In this case, the need for a telescope aperture larger than anything supportable as a monolith within existing launch capabilities necessitates the need for a segmented deployable primary mirror. To collect the desired science, it is necessary to maintain the wavefront to about the 50 nanometer RMS level after deployment. In addition, it is necessary to isolate global telescope alignment errors from the segmented-induced wavefront errors and bring the telescope into a globally optimized alignment. Several techniques have been proposed for sensing the wavefront error in the resulting collected image, with the intent of adjusting the opto-mechanical system to reduce the errors to acceptable limits. In addition, a small set of image examinations have been simulated to determine global misalignments in deployed systems. BATC has developed a testbed to support evaluation of the various techniques for autonomously measuring and correcting wavefront errors and for isolating misalignments in large telescope systems. The testbed is designed to be modular, with separate subassemblies providing the segmented input wavefront, the control capability, and the imaging and sensing capabilities. It is also designed to be an evolving asset, providing several levels of testing enhancements over time and supporting the development of test facilities that will be integral to future observatory integration. This paper describes the initial and future top-level requirements, design parameters, and performance capabilities of the testbed.
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