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January 20, 2018

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Staying shiny

25/11/2002

The Gran Telescopio CANARIAS's 36 mirrors are not just special because they will make up a primary mirror more than 10 meters in diameter, nor because they will be part of the world's biggest telescope.

When they take their place among all the small and large parts that will make up the GTC - like chips, sensors, motors and bearings - these hexagonal mirrors, each 8 centimetres thick, 470 kilos in weight and measuring 1.9 metres from vertex to vertex, will be vitreous ceramic components that will need very special care: they will need to be regularly ‘dusted’ and ‘polished’.

The secondary and tertiary mirrors, together with the 36 mirrors mounted in the complex primary mirror cell, will be cleaned with special products like distilled water, alcohol and foams to maintain maximum reflectiveness and remove any dust that would otherwise build up on them. They will also have to take turns to be ‘bathed’ in aluminium at least once a year.

If the GTC’s primary mirror was monolithic - in just one piece - the aluminium recoating process would take two to three days and would be much more difficult: transporting the mirror, even over short distances, would be risky. In addition, the upper manufacturing limit for monolithic mirrors is only eight metres and so, for the GTC, it was felt that the whole process from transport to operation, needed to be more practical.

The segmented mirrors chosen for the GTC reduce the risk involved in transportation and avoid loss of observing time. The segments can be transported with less risk and are easier to operate than a monolithic mirror.

One of the processes we have mentioned is the aluminium ‘bath’. The process is simple: 6 of the mirrors are ‘spares,’ waiting to take their turn to gather light from the stars. When the moment finally arrives, one of the other mirrors will be withdrawn from the primary mirror cell and sent to the Recoating Plant, a room specially designed for the GTC by German company VTD (Vakuumtechnik Dresden GMBH) where it will go through an incredible experience.

“Mirror, mirror on the wall”

First, the mirror will be withdrawn and placed on a trolley designed to allow it to rotate. This is the only time the mirror will be handled - afterwards, up until the moment it rejoins the primary mirror, it will be anchored to the trolley’s guide rails. Once on the trolley, the mirror will move into the washing room where its existing aluminium coating will be stripped off into a closed circuit.

Now ‘naked,’ the zerodur mirror (zerodur is the vitreous ceramic material the mirror is made of) will move on to the recoating plant which, like a huge vacuum chamber, will pull the trolley along the guide rails into its snug-fitting interior.

The hexagonal mirror will enter the upper part of the chamber face-down. Once in place, it will be subjected to a vacuum (10-6 millionths of a milibar), as the chamber expels as much air as possible. This process alone will take one and a half hours.

Aluminium ‘rain’

Before all of this, the aluminium ‘cart’ will have been readied outside the chamber. A retractable device housed in the lower part of the chamber, the ‘cart’ will be used to prepare and hold the aluminium needed for recoating (the quantity will vary depending on whether the primary, secondary or tertiary mirror is being recoated). (PHOTO “Aluminium holding cart”).

The ‘cart’ is equipped with a number of manually loaded evaporators. Each has a tungsten element, or filament, into which are placed wire-like bars of solid aluminium - each evaporator takes just 1.3 grams. Once the aluminium is loaded the ‘cart’ will move into the recoating plant and the process will get underway.

Into the void!

Once vacuum conditions have been created a second cleaning process will be carried out, this time with ion bombardment - a kind of invisible blasting process that removes impurities. The mirror will rotate twenty times per second during the remainder of the process. During the next five minutes the chamber will take on the colour of the gases being ionised, flooding it with blue clouds of argon or a pink coating of oxygen.

Once the mirrors have been cleaned, the elements in the evaporators will receive a low voltage but high intensity electric charge (10 volts to 1000 amps) that will set off what is known as a ‘plasma discharge’: in around one minute the aluminium will be converted from solid into gas.

The aluminium vapour will be deposited at a speed of 1km/s onto all the surfaces within the chamber - including of course the mirrors, which will keep on rotating until the entire process is completed.

So this is how, once the one-micron-thick aluminium layer has condensed, the mirror will be restored to its original condition.

Meanwhile, its replacement will have made sure the mirror has not been missed. As if nothing had happened the telescope will go on watching the stars, its shine untarnished.

Natalia R. Zelman

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