Testable lightweight telescopes for space

The next generation of larger space optics will need lightweight and deployed mirror systems in order to control costs and fit within current and planned launch vehicle fairings. These will require active control based on wavefront sensing to establish and maintain their optical quality. Such control has been the enabling factor for the current generation of 8 m class ground-based telescopes, whose mirrors are either single monoliths with detailed shape control or have multiple rigid segments with control of relative position. They use actuator densities of typically a few per square meter. For active space systems it will be highly desirable to test the full deployed spacecraft in a vacuum test with a scene simulator, to validate before launch the optical performance of the complete system with its closed loop control systems. To enable such testing, the space mirror system must be designed from the start to work in a 1g as well as zero g environment. The orientation we envisage has the spacecraft system pointed at the zenith, illuminated by a downward beam collimated with reference to a full aperture liquid flat. We consider here two space mirror systems. The first has rigid segments supported by position actuators to control only rigid body motions. Since the segments under test must hold their shape with an axial 1g load and no passive flotation supports, they must be smaller than for ground systems. If made of lightweighted silicon carbide or beryllium for diffraction limited imaging in the optical, they would have to be ~ 30 cm in diameter. A mirror systems made from such segments will require about 40 actuators and wavefront sensor sub-apertures per square meter. The second system is a lightweight 3.5x8 m monolith for very high contrast imaging, as is envisaged for NASA's Terrestrial Planet Finder. High accuracy control of Fourier components down to ~ 0.2 m period is required, requiring a deformable mirror with about 4000 actuators. If the primary itself is the deformable element, and has a 1 cm thick glass meniscus facesheet weighing 600 kg, the gravity-induced quilting during testing would be about 1 nm rms, low enough for ground testing of the complete system at the desired 10-10 contrast level.