5.5. Wavefront Control System#
The Wavefront Control System (WFCS) is the set of TCS software components that implements all active and adaptive optics control for the GMT. It includes all of the software and computing hardware necessary to determine the optical aberrations detected by AO and telescope wavefront sensors and to convert them into corrections applied to the AO and telescope optics. This integrated wavefront control approach is different than that followed by many previous telescope projects in which the AO control system was logically distinct from the telescope control system. This approach, enabled by the concurrent development of telescope and AO systems, avoids the duplication of infrastructure components and facilitates the consistent design of different GMT observing modes, especially when the boundary between telescope and AO becomes fuzzy (e.g., AGWS wavefront sensors are used in some AO modes or the ASM is used in natural seeing mode).
The wavefront control system involves hardware and software components developed in several groups and organizations. A careful and systematic definition of interfaces that evolve with the design is of paramount importance to ensure that, once integrated, the system operates with the required performance and robustness.
The GMT is required to support four observing modes: Natural Seeing, GLAO, NGSAO, and LTAO. A different but overlapping set of wavefront sensors and optical compensators is used to meet the optical performance requirements of each. The WFCS must therefore be modular and reconfigurable, while providing the high computing performance and low latency required for AO control. In the TCS, each observing mode has a distinct wavefront controller, but all modes share some common software components. Each wavefront controller is instantiated at runtime; the connections between software components are established by a wavefront control supervisor component specific to each observing mode. A top-level wavefront control supervisor implements changes between observing modes.
One type of component common to all observing modes is the optics controller. The M1 Optics Controller converts wavefront residuals to segment rigid body motions and bending modes, which are then passed on to the M1 System hardware controllers. The M2 Optics Controller converts wavefront errors such as focus and pointing-neutral tilt to rigid body motions of the M2 segments, which are passed on to the M2 Positioner hardware controller.
A full description of the implementation of the wavefront controller for each of the three AO observing modes is given in the AO Preliminary Design Report [Bouc13]. Only an outline of the Natural Guide star Ground Layer AO observing mode WFC is presented here, providing an example of the interaction between the AO wavefront controller components and the Active Optics Wavefront Controller.