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A tower for a kingdom of mirrors

Standing out sharply against the blue Chilean sky in this Picture of the Week is the central tower of ESO’s Extremely Large Telescope’s (ELT). The size of a three-storey building, the tower looks impressive by itself, but it is only a small part of the superstructure that will be the ELT.

This skeleton frame will support three of the ELT’s five mirrors, all with different shapes and sizes: M3 (at the bottom of the tower), M4 (at the top) and M5 (at the centre). Together they will guide the light that the telescope catches from space all the way to its instruments, which capture the images and data. In this video you can see how the light travels through the telescope.

M4, at the top of the tower, will be the largest adaptive optics mirror ever built. Its surface can be deformed — up to 1000 times per second — to correct for atmospheric turbulence and for possible vibration caused by the rotating telescope structure or by strong winds. This is crucial for delivering the sharp images scientists need for their research.

M5, at the centre of the tower, will redirect the light to the scientific instruments at the side of the telescope. This ‘tip-tilt’ mirror will steer the light up to 10 times per second to stabilise the images.

As the tower with the three mirrors will be attached to the centre of the ELT’s main mirror cell, it has to be light enough to not deform the mirror structure. At the same time, the frame has to be rigid enough to guarantee that the tower will only bend minimally under its weight when the telescope is tilted. And all this while letting wind go through the tower to prevent the formation of air pockets that could distort the images. A delicate balance, and a true feat of engineering!