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October 2, 2023



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The chosen one


No-one said anything, but it was obvious that the atmosphere was getting tense. Some of them could feel the actuators tingling in anticipation at the thought that they could be the next "chosen one".

Finally the members of the optics team approached the transporter that had been specially designed to move the segments; segments which, when joined together, would make up a surface 11 metres wide. Like the winner of a competition, OP-M1-SG-003-0006 of lot 6 (ADERNO to its friends) was taken off its shelf and loaded onto the cart. Its ZerodurTM body, invulnerable to changes caused by heat, nonetheless felt a premonition of what was to come: the aluminizing process.


At the start of the process the selected mirror does not reflect anything; its surface has the same colour and opaqueness as honey.

To be able to reflect, each and every one of the mirrors on this telescope (the segments of the primary, secondary and tertiary mirrors) has to be coated with a fine layer of aluminium.

For the selected mirror, the process begins with a trip to the cleaning room where any remnants of the previous aluminization process are removed.

The primary, secondary and tertiary mirror segments are moved using specially designed transporters, which rotate and are fitted with an anchoring system to keep them in the desired position at all times during the aluminizing process.

After being exhaustively cleaned, the segment is moved to another room to wait for its turn in the the aluminizing chamber, which is specially designed for the GTC and was built by the German company VTD (Vakuumtechnik Dresden GMBH)(). The mirror will be taken from the transporter with its anchoring system and placed face-down in the chamber.


Prior to this, the "aluminium cart" has been readied. It holds the filaments that will contain the aluminium threads and it can be moved into and out of the chamber. The aluminium threads used are 99.999% pure, 2mm in diameter and 130mm in length with a weight of 1.1g. They are folded into a figure eight and inserted into the filaments.

Once the aluminium is in place the chamber is closed with the cart inside it, and the pre-evaporation process begins. This melts the aluminium in the filaments but does not evaporate it.


Now the mirror segment, the filaments and the chamber are ready.

Once the segment is in place, the lid of the aluminizing chamber is closed and a vacuum starts to develop. It will reach 10–6 millibars in approximately one and a half hours. A stream of ions (our segments are familiar with this as they have already been polished by ions) is used to remove any remaining impurities from the mirror surface. The chamber becomes violet in colour as oxygen is ionised during the process, which is called plasma discharge. The flashes that occur are proof that this final stage of the cleaning process is working properly.

An electric current then passes through the interior of the chamber, vapourising the aluminium and depositing it onto the mirror in a fine layer no thicker than one micron.

When it comes out of the chamber, our segment is the star of the hall of mirrors...but not for long. Soon they will all experience the same empty feeling, but it will leave them full of reflection.

The Aderno Tree

The Aderno is a tree native to the laurisilva slopes and other humid forests of the Canaries, although it is not common and is completely absent from Lanzarote. Few species of its kind reflect the ground they grow on to such an extent: the aderno grows no bigger than a shrub when it is on cliffs and precipices, but can reach heights of six metres on laurisilva slopes. Only at Mazizo de Teno, in the far northeast of Tenerife, can large amounts of this hard wood tree, which was often used for furniture making, be found. Furniture is one of the few uses of this indigenous species, whose closest relatives are in the tropical zones of Asia and America, but its fruit, dark cream in colour when ripe, has a sweet flavour. The historian Juan Bethencourt Alfonso refers to the word 'aderno' as part of the heritage of Tenerife and La Gomera, although he describes it only as 'a tree with very hard wood', which can be found in the transition zones between different types of forest.

Text: José Manuel Abad Liñán

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