The function of the ELT’s telescope structure is to support the optics and the instruments while tracking astronomical objects. For this reason the ELT’s design was driven by both the need to point and track the sky as well as excellent image quality. To achieve this, the entire optical path and the instruments must be kept stable under all operating conditions, including changing temperatures and strong winds. The challenge in designing the ELT is to provide a sufficiently stiff structure for the primary mirror segments and the other optical elements, while not dramatically increasing the weight of the structure, which would limit its dynamical performance.  

The design had to consider not only the astronomical performance, but also the fact that the telescope must survive Chile’s regular, strong earthquakes, without subjecting the delicate optics to strong accelerations. The earthquake protection system is located below the telescope structure. The telescope pier and the dome pier are mounted on foundations that are structurally separated, to avoid possible propagation of vibrations from affecting the image quality.  

The telescope structure is an alt-az mount and is divided into two main parts: the azimuth structure and the altitude structure.

The azimuth structure supports the ELT telescope tube – or altitude structure – and the scientific instruments. The scientific instruments are positioned on the two Nasmyth platforms, each with dimensions of approximately 30 m by 15 m, a size that allows each platform to host a pre-focal station and three large instruments. A truss structure links the platforms to the ring-shaped azimuth floor. The azimuth floor covers the various mechanisms (bearings, motors and encoder) and provides a large surface for handling the telescope and access to it.

Both the azimuth and altitude axes rotate the telescope using hydrostatic oil bearings. In the azimuth axis, the telescope is rotated using these bearings arranged on three concentric tracks, and one radial track which defines the azimuth rotation axis. The largest concentric track has a diameter of 51 m, with the intermediate and central tracks having diameters of 34 m and 6 m, respectively. The tracks are bolted to the telescope pier to provide structural stability. This arrangement allows a direct load path between the azimuth structure and the pier, and provides stability to the instruments and sufficient stiffness to limit the effects of wind on the telescope. The tracks have to be aligned to a precision of just a few tenths of a millimetre over their full diameter. The dimensions of the central radial track allow lateral loads to be absorbed when earthquakes occur. 

The azimuth floor allows access to the ELT telescope to install its components, with a bridge crossing the floor’s diameter below the telescope tube. The overhead crane installed in the dome allows positioning of the optomechanical elements on the azimuth floor. From here, the primary mirror (M1) segments can also be lifted for installation in the M1 cell. The optical elements within the central tower are moved on rails along the azimuth bridge, before they are lifted into their final position.  

Linear motors will drive the movement of the ELT. These are distributed all along the circumference of the external azimuth track. The azimuth structure is equipped with two encoders to detect and feedback on the telescope’s motion in the azimuth axis: one at the centre used for the position loop, and one at the external diameter close to the motor, to close the velocity loop. Extensive studies have shown that this solution has the best potential for fulfilling the stringent pointing and tracking requirements because it helps overcome the dynamical deformation of the structure, which is unavoidable for telescopes the size of the ELT.

The altitude structure hosts the telescope optics, including the ELT’s five mirrors. It is composed of the enormous open telescope tube structure, which supports and connects the 39-metre primary mirror and the secondary mirror hanging above it, and the central tower. The telescope tube is structurally closed at the bottom by the M1 support structure (the M1 cell) and by the top ring, ‘spider’ and secondary mirror crown at the top.

The altitude structure rests on hydrostatic pads, arranged in two groups on each side of the telescope, creating a sort of cradle. Cylindrical tracks mounted on two semicircular plates attached to the M1 cell rest on the hydrostatic pads. The plates are perforated to allow air flushing of the primary mirror when the wind comes from a lateral direction, helping minimise air temperature differences between the primary mirror and ambient air.  

The M1 cell supports the primary mirror and is a truss structure consisting of three levels. The upper part is the frame directly supporting the 798 mirror segments constituting the M1 mirror. The upper surface of the M1 cell contains openings to access and service the delicate components of the M1 segment supports, which shape the mirror’s configuration. Below the level of the M1 segments there is a walkable area used to access the location of each segment via circumferential and radial corridors. This access allows segments to be extracted for recoating and to service the M1 segment control electronics. Below the walkable plate another lattice structure provides stiffness to the M1 cell. This stiffness is needed to support the central tower of the telescope which is directly mounted at the centre of the M1 cell.   

The telescope tube itself is a truss structure constructed with tubular profiles. The short focal length of the primary mirror results in a tube structure length of approximately 27 m. At the top of the tube, the top ring has a polygonal section optimised for wind resistance, manufacturability and structural performance. The M2 crown that hosts the M2 mirror unit is connected to the top ring by means of six beams, forming the ‘spider’. When all elements are considered, the total length of the altitude structure, from the bottom of the M1 support structure to the M2 crown exceeds 50 m. The Nasmyth platforms are located 27 m from the ground and the altitude axis is at a height of 33 m. When the telescope is pointing vertically upwards, the M2 crown reaches 60 m above the ground.  

The linear motors of the altitude structure are arranged so that the magnets are mounted on the altitude structure and the coils of the motor are on the azimuth structure. Like the azimuth axis, the altitude axis is equipped with two encoders, one close to the motors, used for the velocity loop control and a central one for the position control loop.