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February 5, 2023



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The magnificient seven


A week has seven days and a rainbow has seven colours; the world has seven wonders and the GTC has seven foci. Many people believe that the number seven is magical - and we agree. After all, seeing stars, planets and galaxies through a telescope is a kind of 'magic'.

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It is a magic trick in two halves - first there must be light and second one of the GTC’s seven foci. Together they give an image of an object millions of light years away across the universe.

Isn’t that incredible? You will never count to seven again without thinking of the GTC!


To begin with, light from the stars bounces off the primary mirror. It is deflected to the secondary mirror which, when required, can send it on to the tertiary mirror. This is when the foci take over. The foci are the points at which the beams of light reflected by the mirror, or principal lens, converge, and they are the physical locations at which the instruments used to analyse the light are mounted.

1 “Cassegrain”, 4 “folded Cassegrain”, 2 “Nasmyth” - these are the GTC foci, all of them named after their “creators”.

Cassegrain, an astronomer, medic and sculptor, was a member of the court of Louis XIV and was the second person to design a working reflecting telescope. In his model, light was reflected from a concave primary mirror to a convex secondary mirror, which then reflected the light back on to the focus through a hole in the primary mirror. The system is still used today, particularly for large telescopes like the GTC.
Folded Cassegrain foci are the same as normal Cassegrain foci, except that they are located inside the elevation ring and the light that they work with reaches them from the tertiary mirror.

Scottish engineer James Nasmyth developed the perfect system for using heavy instruments without affecting the balance of the telescope: he added platforms, mounted in the wings of the telescope, which ensure that the image is always steady.

The GTC is made up of an endless number of parts, but the most important one for the foci is the tertiary mirror. This determines which of the Nasmyth or folded Cassegrain foci the beams of light go to. The Cassegrain focus, in contrast, needs no assistance from the tertiary mirror - light comes to it direct from the secondary mirror.

Each one of the foci is assigned its own functions, which are allocated according to the weight that they can carry - in turn determined by size, volume and a number of other factors.

The Cassegrain focus can carry a total of 5100 kg, whilst the folded Cassegrain foci can only carry smaller instruments.

The instruments mounted at the Nasmyth focus are much heavier. Its platforms can carry loads of up to 1,500 kg/m, which means that other equipment, including the resistance and control units for the rotator, can be mounted on them in addition to the instrument itself. The two Nasmyth platforms are identical and both of them have the same purpose: to carry heavier instruments.

Seven - the number of islands in the Canaries, each with its special features and its “magic”, and the number of foci at the GTC. Light will wend its way across each of the foci, generating images of stars, planets and galaxies...and each time the light hits a focus and is captured it is not just incredible views of the sky that we will see: the very mysteries of the Universe will be unveiled before our eyes.

Text: Oliver Expósito


    • Total mass = 5.100 kg
      • 2.000 kg instrument
      • 11.100 kg acquisition and guide unit
      • 1.200 kg instrument rotator
      • 800 kg instrument and adaptive optic calibration module
    • Volume
      • height = 2,36 m
      • diameter = 1,99 m

    • Total mass = 1.000 kg each
    • Volume
      • length = 2 m
      • diameter = 1 m

    • Weight of instrument when attached directly to the rotator = 2.400 kg
    • Weight of the instrument when a second support bearing is used = 7.500 kg
    • Volume
      • length = 6,135 m
      • width = 5 m
      • height = 4,5 m

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