The Next Generation Space Telescope (NGST) with its large light-gathering mirror and superb resolution will be capable of detecting faint signals from the first billion years, the period when galaxies formed. CSULA's Precision Segmented Reflector Program, sponsored by NASA under the IRA program, serves as a first step in the development and validation of the enabling technologies that ultimately will be used by the NGST mission in 2009.
The proposed research will build and expand on the existing IRA program currently under way at the SPACE (Structures Pointing And Control Engineering) laboratory at CSULA. Advanced technologies for fault identification, precision pointing, and reconfigurable control will be developed and experimentally validated on the SPACE testbed. Due to the nature of the structure, the research will employ decentralization techniques for the development of control laws to accomplish precision pointing with an accuracy of 2 arc seconds. Design of control laws will be based on various approaches including robust control and neural networks. A system identification task will be responsible for tuning the dynamic models used for controller design.
The testbed is designed to emulate a Cassegrain telescope of 2.4-meter focal length with performance comparable to an actual space-borne system. The system's top-level requirements figure maintenance of the primary mirror to within 1 micron RMS distortion with respect to a nominal shape of the primary mirror, pointing accuracy of 2 arc seconds, a high level of disturbance rejection (100:1), and attenuation of vibration due to gravity, thermal and seismic effects, and control structure interaction.
The primary mirrors
The primary mirror is composed of a ring of six actively controlled hexagonal panels arranged around a central panel. The central panel is fixed and serves as a point of reference to the moving panels. The testbed is a control-oriented experimental system and due to the difficulty and added expense of actual optical quality segments made from glass, the panels are made of aluminum honeycomb plates. The required paraboloid surface is thus maintained by positioning the flat panels as tangents to the surface.
The supporting truss
The primary mirrors are supported by a specially designed lightweight truss structure whose structural dynamic characteristics are representative of a large, flexible space-borne system. This includes low frequency modes, high modal density and global mode shapes that properly reflect the coupling of the sub-elements of the structure. In the design, a careful trade-off between the need for the structure to support itself in the 1-g laboratory environment versus the need to keep the frequency of the first mode as low as possible was achieved by employing multi-criteria optimization technique and Pareto optimality concept.
The isolation platform
The entire testbed is supported on a triangular isolation platform made of aluminum honeycomb core with stainless steel top and bottom skin. The table is made of an aluminum honeycomb core with a stainless steel top and bottom skin. It is mounted on three pneumatic cylinders providing a passive isolation system for the entire testbed.
The secondary mirror
The SPACE testbed active secondary mirror is attached to the primary by a tripod. This critical feature has been added to the testbed to make its performance and functionality more comparable to an actual space-borne telescope. It has been designed to provide three-axis active control, with control system hardware that consists of a number of reluctance actuators and position sensors that move and control the secondary mirror.
The secondary mirror is a six-sided pyramidal aluminum mirror located 2.4 meters above the primary mirror. The mirror has a diamond fly cut in its reflecting surface at an angle that allows it to split the incoming laser beam into six sub-beams. A series of springs suspend the secondary mirror from the secondary mirror housing. The secondary mirror has three degrees of freedom in the form of tip, tilt and piston motion. It is equipped with 3 reluctance actuators to provide 3-axis active control and 3 inductive position sensors that provide information about the secondary mirror attitude.
The optical scoring system
The SPACE testbed is fitted with an optical scoring system consisting of the secondary mirror, a laser source placed in the center of the primary, and an array of optical sensors that will detect any deviation from a reference position. The testbed's control system is composed of a data acquisition system, computers, and actuators.
A Helium-Neon laser source in the center of the primary mirror aims a calibrated laser beam at the secondary mirror. The secondary mirror has a six-sided pyramidal design that splits the incoming laser beam into six sub-beams and directs them to each panel. Small one-inch mirrors mounted on the edge of the panels return each sub-beam back to the secondary mirror which in turn reflects the beam back to the optical sensors located at the center of the primary mirror