Abstract
Although large cryogenic space telescopes may provide a means of answering compelling astrophysics questions, the required increase in the primary mirror diameter presents technical challenges. Larger primaries are more flexible, and cryogenic mirrors are typically very lightly damped—the material damping is negligible, and common damping methods break down. To address these challenges, we propose placing flux-pinning mechanisms along the edges of adjacent mirror segments. These mechanisms consist of a collection of magnets and superconductors, and like flexures, they preferentially allow motion in specific degrees of freedom. Motion in nonpreferred degrees of freedom is resisted by a force analogous to a damped spring force, and the stiffness and damping can be adjusted independently. As an example, we consider simple mechanisms consisting of an inexpensive magnet and a single superconductor. These mechanisms provide increasing resistance as the magnet and superconductor—or mirror segments attached to each—come closer to colliding. These mechanisms, with typical stiffness and damping values on the order of 5000 N/m and 5 kg/s, respectively, also provide modest improvements to the mirror performance. Greater gains can be achieved by using stronger magnets or smaller separations, or by placing nonmagnetic conductive materials near the mechanism.
Highlights
Large cryogenic space telescopes may provide a means of answering several compelling astrophysics questions, but the required increase in the primary mirror diameter presents numerous technical challenges
One approach to increasing the mirror stiffness and damping is to use an edgewise-connected architecture, with flux-pinning mechanisms placed along the segment edges
Consisting of a configuration of magnets and superconductors, flux-pinning mechanisms are uniquely suited for cryogenic mirrors since they require low temperatures to operate, unlike mechanical devices, which can have problems with lubrication, coefficient of thermal expansion (CTE) matching, and thermal snap
Summary
Large cryogenic space telescopes may provide a means of answering several compelling astrophysics questions, but the required increase in the primary mirror diameter presents numerous technical challenges. Large stiff precision structures are considered an enabling technology for large cryogenic mirrors,[12] an alternative approach to increasing mirror stability is to use an edgewiseconnected architecture In this approach, mechanisms analogous to damped springs are placed along the edges of the primary mirror segments. Unlike mechanical devices, which can have problems with lubrication, coefficient of thermal expansion (CTE) matching, and thermal snap, flux-pinning mechanisms operate best at cryogenic temperatures These passively stable, noncontacting mechanisms consist of a collection of magnets and type II superconductors and require only low temperatures; no power is needed other than the minimal amount, if any, necessary for cooling. Motion in the nonpreferred degrees of freedom is resisted by a force analogous to a damped spring force, and the stiffness and damping can be adjusted independently These mechanism properties depend on the choice of magnets, the separation between the magnets and superconductors, and the presence of nonmagnetic conductive materials, such as aluminum. Greater gains can be achieved by using stronger magnets or smaller separations, or by placing nonmagnetic conductive materials near the mechanism
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