5. Telescope Control System¶
The Telescope Control System (TCS) encompasses the subsystems in charge of the control of the telescope optomechanical devices. The design considerations of the TCS provide a solution that seamlessly unifies the different modes of operation: natural seeing, ground-layer AO (GLAO), laser tomography AO (LTAO) (see System Level Requirements in Table below), into one system that is coherent, easy to manage, maintain, and upgrade.
| Title | Statement |
|---|---|
| Natural Seeing Mode | Provide natural seeing observing mode that will be operative with FSM or ASM. |
| GLAO Observing Mode | Provide GLAO observing mode in which the light of astrophysical sources is corrected using multiple guide stars. |
| NGSAO Observing Mode | Provide NGSAO observing mode in which the light of astrophysical sources is corrected using a single natural guide star. |
| LTAO Observing Mode | Provide an LTAO observing mode in which the light of astrophysical sources is corrected using multiple guide stars and one or more natural guide stars. |
The TCS subsystems are organized in a control and supervisory subsystem hierarchy that mirrors the physical arrangement of the telescope subsystems. At the bottom of the hierarchy, device control subsystems take care of the control and operation support for the independent telescope hardware devices. These devices are developed following different timelines and procurement paths (e.g., M2 positioner, M1 control system, Mount control system). This separation is similar to the one used in other telescope projects and provides a modular design that facilitates interface definition, development planning, and integration and commissioning activities. Often these subsystems will have an independent Factory Acceptance Test process prior to integration at the observatory. Each device control subsystem follows the reference architecture as described in Device Control Framework. In addition, to the device control subsystems – mainly concerned by mechanical degrees of freedom – two subsystems provide high-level coordination and control functions: the Pointing Kernel and Wavefront Control System (WFCS).
The Wavefront Control System provides the high-level optical control of the telescope. The Pointing Kernel relates sky coordinates with mechanical and detector coordinates and vice versa. The wavefront control and pointing kernel subsystems coordinate these low-level device control subsystems to point the telescope and move probes to the corresponding targets, to achieve and maintain the nominal optical configuration, and to close the wavefront correction loops during the execution of an observation. Observations and other operations are defined and automated using the tools provided by the observatory operations system (e.g., observing tools, sequencer).
Although the most critical function of the TCS is the control of the hardware devices, efficiency, safety, and robust operations are also essential to operations, as outlined in the System Level Requirements.
| Title | Statement |
|---|---|
| GMT Observing Efficiency | Design to optimize on-sky observing efficiency. |
| Design Safety | Adopt design safety practices to reduce risk to personnel and equipment. |
| GMT System Health | Provide continuous performance, status, and system health monitoring. |
| Flatfield and Spectral Calibration | Provide deployable flat-field and spectral calibration systems for natural seeing and AO instruments. |
| AO Calibration | Provide a system for calibrating all deformable mirrors and wavefront sensors required for each of the AO observing modes. |
| Optical Alignment | Provide equipment and procedures for aligning all optical systems to meet performance specifications. |
| Engineering Data Systems | Provide an engineering data system to monitor the health of all subsystems critical to the functioning and performance of the observatory. |
| Diagnostic Software | Provide software tools for displaying real-time and long term trends in the performance of individual components/subsystems and to correlate that information with time-stamped data from other subsystems. |
The above considerations lead to a design of the TCS that also has the following functionalities:
- Calibration – In many subsystems, calibration software is required to produce error maps or look-up tables that allow the control system to perform with the necessary accuracy.
- Diagnosis – Some of the GMT hardware subsystems are quite complex. Specific software components are often required to verify the behavior of the system or to troubleshoot it in case of incorrect behavior.
- Safety – There are some scenarios where a hardware device could be damaged due to incorrect operation. The Interlock and Safety System (ISS) is ultimately responsible for preventing any damage to equipment or injury to people by the telescope. The ISS may “kick-in” in a drastic way. Safety control components implement a safe operation that avoids the engagement of the ISS.
- Supervision – Often the operation of a subsystem involves the coordination of several subordinate components. It is then the TCS’s responsibility to guarantee that subordinate components are in the correct state, and that the performance degrades gracefully in off- nominal scenarios. In addition to managing the configuration, startup, and shutdown of the control subsystems and hardware devices, supervisory components also implement kinematic transformations or anti-collision protection strategies when the system under control requires it.
The sections below give an overview of the TCS. TCS Subsystems provides a summary of the subsystems inside the TCS, followed by the degrees of freedom that the TCS controls in Degrees of Freedom. The Requirements on TCS, Pointing, Tracking & Guiding section presents the requirements governing the TCS design and operations, followed by details of the Pointing Kernel. Lastly, the Section on Wavefront Control System (TBC…) briefly describes the architecture of the GMT Wavefront Control System (WFCS), and the specific designs of the GMT Natural and AO observing modes.