To achieve diffraction-limited seeing with the next generation of giant telescopes will require a multi-tiered approach employing both active and adaptive optics. Before removing atmospheric effects via adaptive optics, the telescope structure must be actively controlled to remove structural dynamics effects from the optical wavefront. While low frequency thermal and gravitational effects may be removed by the primary mirror control system, a more difficult challenge is the higher frequency, wind-induced vibrations of the telescope structure. This paper will address the control system design for the rigid secondary mirror support structure using modern control methods. Multivariable control methods are motivated by the large number of coupled structural modes that contribute to the wavefront error at the secondary mirror. H2 methods are applied to the secondary mirror control system in order to validate the design approach for achieving nominal performance at the system level. The approach investigated for this paper involves using wavefront information to remove wavefront error at the secondary mirror. This work will serve as a basis for demonstrating the feasibility of the overall control architecture for the GSMT point design study.
The next generation 30-meter class ground-based telescopes pose an unprecedented challenge for control systems envisioned to support diffraction limited imaging. Our approach is a multi-tiered, decentralized control architecture utilizing two kinds of feedback: optical and mechanical. Based on simulations, we suggest a configuration where the optical feedback loops for the main axes, secondary mirror rigid body motions and segmented primary mirror are separated in both temporal and spatial frequency domains. The active optics system maintaining the continuity and higher order shape of the primary mirror is based on mechanical sensors while the low order shape is corrected through optical feedback. Shown are the results of numerical simulations using real, measured wind data that prove the feasibility of the suggested architecture.
This paper presents a Navier-Stokes based numerical methodology for 3D unsteady viscous flow over a general mirror configuration. In this approach, the Reynolds averaged Navier-Stokes equations in finite volume form are solved unsteadily using Roe’s scheme on body fitted H-H type grid. A two-equation turbulence model was implemented. This method is three or five order accurate in space and first order accurate in time. The physical space modeled is a 120m×120m×120m region enclosing the primary mirror at the center. The inflow turbulence from the enclosure openings is represented by turbulence kinetic energy at the inflow boundary. The wind buffeting effect was studied by directly calculating flow field response to buffeting impinging wind. The unsteady pressure distribution on the mirror is extracted and analyzed for the amplitudes and frequencies of dynamic wind loading. The computational results are visualized to highlight the flow pattern, particularly on the mirror upper surface. Results are presented for a 30-meter aperture GSMT primary mirror. A 1:833 model of the GSMT primary mirror was tested in a water tunnel, in which the velocity distribution was measured using PIV technique. The preliminary experimental observation serves as qualitative validation of the simulation capability.