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September 20, 2017


All about GTC

How will it work?

The mirrors

Primary mirror

Achieving a primary mirror diameter of 10.4 m would be very difficult if the mirror were of a single piece. Not only is the size of single mirrors limited to about 8 m by the constraints of available technology, but also transporting and moving one as big as the GTC’s would be very difficult. So for the GTC a segmented mirror was chosen, with 36 pieces.

The GTC, one of a generation of segmented primary mirror telescopes, which also use thin mirror technology to keep down weight. This is a technique first used on the Keck telescopes in Hawaii.

The size of the primary mirror is very important. The bigger it is, the more light it collects. Image clarity or quality (angular resolution) goes up in relation to the diameter of the primary mirror, whilst photon capturing ability goes up in relation to its surface area (the diameter²). This combination - collecting more light with better spatial resolution - makes telescopes like the GTC essential for astronomical research in the years to come.

The primary mirror designed for the GTC is like a jigsaw puzzle, the pieces of which must move together in a perfectly synchronised way. Made up of 36 hexagonal ceramic mirrors - each 8 cm thick, 470 kg in weight and measuring 1.9 m from vertex to vertex, the primary mirror will always function as a single, quasi-hexagonal surface. Measuring 11.3 m across (the same size as a 10.4 m diameter circular mirror), there will be a gap of less than 3 mm between each of the primary mirror’s segments. In all, it will weigh about 16 tonnes just by itself! 

At first sight, these hexagons will all look the same, but in fact their shapes will differ slightly in a way that makes a hyperboloid – a very wide bowl – when they are put together. The 42 mirrors (36 plus 6 spares) are being manufactured in Germany and will be polished by a French company until their surfaces conform to a 15 nanometre (millionths of a millimetre) margin of error. In other words they will have a surface that is accurate to within one three thousandth of the width of a single human hair!

The segments are made of Zerodur™, a ceramic material used in hobs, which is characterized by its low coefficient of linear thermal expansion. This means that the segments will expand and contract very little so that alteration in the mirror's shape due to temperature changes will be barely noticeable.

Putting together the 36 mirrors, each individually shaped, to make a perfectly concave hyperboloidal surface, is not easy. It will be done by computer-controlled ‘actuators’. Unevenness in the primary mirror caused by the way the segments are put together must be kept to less than 90 nanometres. In an area the size of Spain and Portugal this would be like having ‘mountains’ just one millimetre high!

Sensors that send information every two seconds will be used to calculate the margin of error that needs to be corrected for the mirrors to be perfectly aligned. This technique is called active optics.

The mirrors will be cleaned regularly with special products such as distilled water, alcohol and foams, to remove any dust that would otherwise build up on them.

A telescope’s mirrors have their reflective coating renewed too by steaming them in aluminium vapour so they stay as reflective as possible (this is called optical recoating). This will take place in a special room in the telescope enclosure called the recoating plant.

This room will be a vacuum chamber in which the segments of the primary mirror will be coated with aluminium. To be recoated, a segment will be taken out of the primary mirror (one of the spares will replace it) and placed in the chamber. Once the segment is inside, aluminium vapour will coat all of the room’s surfaces, including the mirror. The metal will be vaporized by passing bars of aluminium over an electric element, so that their temperature rises in a few seconds and they turn from solid into gas.

Remember that no observing time will be lost while these procedures are being carried out. Since the mirror is segmented, while one segment is being recoated, another one can take its place, so that work can carry on.  In a telescope with a monolithic mirror the recoating process, apart from being very complex, can put the telescope out of operation for two or three days.