One of the innovative features of the GTC is its advanced control system. This is the ‘brain’ that will control all of the subsystems, which have to work together in a coordinated way.
The physical elements of the control system will be distributed throughout the GTC, and will consist of a series of interconnected computers, electronic devices, sensors and actuators. Together they will exercise direct control over the GTC’s various subsystems.
The control system’s job is to monitor and control these subsystems and to provide an integrated user interface, a single point where all of the information being received can be accessed so that it is easier to work with.
The control system will be used for such important tasks as planning observations and archiving and checking the quality of the data. For this reason a number of workstations will be installed and connected via one or more local area networks, giving access to centralized services such as catalogues and archives.
Independently of the complexity inherent in controlling a large facility like the GTC, one of the factors that influenced the design of its control system was the deployment of new technologies and the flexibility to keep them up to date. The approach adopted will allow for constant updating as technology evolves throughout the GTC’s working life.
MODULES OF THE CONTROL SYSTEM
Inspector: this will allow the telescope to be operated from either the control room or the base (at sea level) by astronomers or specialist engineers. It will facilitate uniform access to all of the information collected by the telescope.
Scheduler: a tool that will be used by help astronomers plan observations their over the long, medium and short term. Short term planning will allow atmospheric conditions to be responded to in real time.
Observation Proposal Management System (OPMS): a tool that will be used by astronomers to define proposals for observations that they wish to carry out at the telescope. The definitions produced will be used by the Scheduler to allocate observing time to the proposals accepted.
Sequencer: will allow observations to be carried out automatically using the definition produced with the OPMS. The Sequencer will coordinate all of the subsystems so that they work together to carry out the functions needed to obtain the scientific data that go with an observation.
Data Factory: this system will allow all of the data - whether scientific or related to engineering - obtained by the telescope to be processed so that scientific analysis of it can begin. Once it is up and running, it is estimated that the GTC will generate over 1 terabyte of data per year.
Data Archive Management System (DAM): astronomers who have completed an observation will use this to access and view data from the GTC. After their period of exclusive rights to it has lapsed, the data will become available for consultation by other astronomers.
Common Services: will provide the basic infrastructure needed for the other control subsystems to work, for example alarm management, message management and configuration management.
System Supervisor: will monitor the computers and systems carrying out the control functions to ensure that they are working correctly. Will also allow the whole of the telescope to start and stop automatically and to switch between maintenance and observation modes.
Interlock and Safety System:will prevent dangerous situations occurring owing to the interaction between different subsystems.
Observing Engine: will allow the telescope to be pointed precisely and to track moving objects, using models of how stars move across the sky. At the same time, using information from the acquisition and guiding control system, it will calculate the optimal position for the different optical components, taking into account structural deformation due to gravity and temperature fluctuation.
Main Axis Control System (MACS):will control and monitor the azimuth and elevation axes, field rotators (one per focal station), hydrostatic bearings and counterweights. A network of temperature sensors will allow the way the telescope is operating to be monitored and adjusted as the temperature fluctuates.
Dome Control System (DCS):will control and monitor the dome’s rotation, the dome shutter, the windscreen and the 32 ventilation apertures.
Civil Works Control System (CWCS):will control and monitor the temperature in the telescope chamber, the enclosure air-conditioning system, and the condition of the electrical equipment.
Primary Mirror Control System(M1CS): will control the following field elements: 216 sensors and 108 actuators, which will adjust the position of each segment relative to its adjacent segments; 216 load cells and 216 motors, which will adjust the unique shape of the segments; 216 temperature sensors, which will monitor thermal performance. In all, 972 field elements will ensure that the mirror’s surface is in optimum condition. This system allows the position of all of the segments to be monitored 200 times per second and to be adjusted 50 times per second.
Secondary Mirror Control System (M2CS): will control a five-degrees-of-freedom hexapod robot. This system will move 40 times per second to compensate for the movement of the stars.
Tertiary Mirror Control System (M3CS): will control the rotation and “parking” movements of the mirror, needed to deflect light received from the secondary mirror to the different foci.
Acquisition and Guiding Control System (AGCS): there will be one AGCS for each focal station. They will control the motors for the two turntables and the 2 articulated arms that will be used to position a CCD sensor in the telescope’s field of view. The arms will be equipped with optical sensors that will provide, after the necessary processing, the information needed for active control of the telescope’s optical system. For example, a star’s movement will be measured 200 times per second so that the movements the secondary mirror needs to make can be calculated.
Commissioning Instrument Control System (CICS): will control and monitor the mechanisms of the checking instrument and the data acquisition system.
(ICMCS, Instrument Calibration Module Control System): will control the switch on-switch off of the calibration lamps and the movements of a parabolic mirror used to calibrate the scientific instruments.
(EMCS, EnvironmentalMonitoringControl System): will control and read all the humidity, temperature and wind sensors installed into the telescope area. It also controls and reads the meteorological station sited outside the GTC.
Instrument Control System(ICS): the mechanisms of every instrument will be linked to the control system and the data acquisition system.
Together these systems will make up a network of approximately 35 computers, with associated electronic components, and 15 work stations and servers, which will all be linked together by several kilometres of optical fibre and a sophisticated communications system.