European
Southern
Observatory

Telescope
Controls

The control system of the ELT allows the user to operate the telescope for science observations and maintenance activities.

It consists of computing hardware, sophisticated software, and communication infrastructure that bring together the many components of the ELT and ensure it is used in a safe and coordinated way.

In a nutshell

It consists of computing hardware, sophisticated software, and communication infrastructure that bring together the many components of the ELT and ensure it is used in a safe and coordinated way.

The control system of the ELT allows the user to operate the telescope for science observations and maintenance activities.

It consists of computing hardware, sophisticated software, and communication infrastructure that bring together the many components of the ELT and ensure it is used in a safe and coordinated way.

The many advanced mechanical and optical components, or subsystems, of the ELT, are operated by means of a control system. This complex system integrates the various components of the telescope and works as the user interface to the ELT. It provides coordinated, safe operation of the subsystems as a single system to perform science observations and support engineering and maintenance activities. 

The telescope control systems are also key in helping the ELT obtain sharp images. The complex primary mirror control system monitors and corrects the positions of its 798 segments, which are distorted by the effects of gravity and temperature, so it continues acting as one mirror. The M4 and M5 mirror control systems are important for compensating for distortions in the images caused by atmospheric turbulence and wind, an important part of the ELT’s adaptive optics.

Control system overview

Overall, the control system manages around 15000 actuators and over 25000 sensors distributed throughout the telescope and dome.  

The communication infrastructure allows the majority of the computing units to be hosted in the computer room in the auxiliary building of the dome. This reduces vibration and heat pollution in the telescope area and provides a controlled data centre environment for the hardware. 

The control system itself is a software-intensive system of systems. In designing such a system, decisions on algorithms and computational performance are not the only major problems. The organisation of the overall system, its behaviour and interactions present a complex challenge.

Layers of the control system

The control system is divided into two layers. The lower layer contains all the local control systems (LCSs), a set of control systems each managing performance and safety for a telescope subsystem. Examples include the LCS for the main structure, the dome, the primary mirror and the lasers, and so on. 

The upper layer is a single system called the central control system (CCS), which is responsible for system level functions and safety. The many control loops, involving both electromechanical sensing and on-sky sensing, are combined and considered as an integrated control problem by the CCS (specifically the telescope realtime executor [TREx]). 

The CCS provides the interface to the instrument control systems to carry out science observations. The instrument control system requests the CCS to acquire a celestial target, and the CCS decomposes the request into operations on individual subsystems: presetting the main structure to point to the target, rotating the dome, configuring the mirrors for best performance given the target location and presetting the guide probes to acquire the guide stars to further correct the telescope’s optical performance. 

The M1, M4 and M5 subsystems

The primary mirror (M1) local control system is one of the most complex subsystems in the telescope. The M1 control system monitors more than 4500 edge sensors on the 798 mirror segments, which provide information on the relative displacement of adjacent segments. It then sends segment position corrections to the three mirror segment position actuators (PACTs) located under each segment. This control, monitoring and correcting at 500 Hz (500 times per second) actively maintains a continuous surface of the primary mirror to an error of around 50 nm. It also isolates the mirror segments from the cell’s back structure which deforms up to 5 mm under gravity, wind and changing thermal conditions. Further segment control is enabled through the M1 warping harness, a set of stepper motors and strain gauges, attached to each segment support structure, capable of controlling surface deformations in the individual segments.

 The ELT quaternary mirror (M4) is a 2.5 m deformable mirror with over 5000 voice coil actuators providing a fast dynamic response. Its local control system maintains the mirror position through a closed-loop control. This mirror receives optical corrections from instrument and telescope adaptive optics modules at 1000 Hz and converts these modal amplitudes to actuator position commands. Movement of the actuators deforms the very thin M4 mirror, so its shape compensates for telescope aberrations and atmospheric turbulence, improving the image quality. 

The next mirror in the light path, M5, is a flat, fast-steering (tip-tilt) mirror used to stabilise the image primarily against wind disturbance on the telescope. Its local control system maintains the mirror position responding to tip-tilt demands from the central control system. M4 and M5 form the core of the telescope’s adaptive optics. 

During observation, the TREx, a component of the CCS, combines wavefront corrections and optical metrology from telescope guide probes and instrument-based sensors, with offload commands to achieve optimal stroke management in the telescope, i.e. maximising the travel-range of the actuators needed to adjust the ELT’s mirrors. By this, the fast-deformable mirror M4, which is continually adjusted to correct for the atmospheric disturbance seen by instrument sensors, has its full stroke arrange always available and optimised by offloading corrections to other (slower) mirror units and actuators in the telescope.