Active correction technique for segmented sub-mirrors using multi-point radial-axial hybrid force actuation
Abstract:
To advance surface error correction and high-precision surface shape maintenance in large-aperture segmented telescopes, this study introduces the multi-point radial-axial hybrid force-driven active correction technique (MP-RHFACT) for sub-mirrors. In contrast to conventional sub-mirror correction technologies (e.g., warping harness), the proposed technique demonstrates a significantly enhanced capability for correcting low-order surface shape errors, such as defocus and astigmatism. The active correction system mainly consists of 12 sets of edge hybrid force actuator assemblies and 1 set of central axial force actuator assembly, achieving active correction of low-order aberrations by coordinating 6 pairs of radial forces and 13 sets of axial forces. For a hexagonal sub-mirror with a circumscribed diameter of about 1.1 m and a vertex curvature radius of approximately 10.6 m, simulation analysis reveals that the technique reduces defocus error by a factor of 85.4, 0°/45° astigmatism by factors of 93.8/94.5 respectively, and horizontal/oblique trefoil aberrations by factors of 16.3 and 8.9, thereby demonstrating excellent correction performance-particularly for defocus and astigmatism errors. It should be noted that the reduction factor is defined as the ratio of the initial surface error RMS to the residual RMS after correction. This method offers robust support for achieving and maintaining high-precision co-phasing in segmented systems. Further investigations indicate that, even in the presence of correction force magnitude errors (±0.2 N) and directional errors (inclination angle ±1°), the RMS values of sub-mirror surface shape error induced by these deviations remain below 6.9 nm, confirming the system's robust performance. In summary, the MP-RHFACT proposed in this paper reduces the complexity, fabrication, and alignment challenges, and manufacturing costs associated with sub-mirrors, thereby providing an effective solution for the engineering application of large-aperture segmented telescopes.
