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October 2, 2023



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The spinning tower


Lots of things can be used to reflect light - like glass, metal...or a mirror. The GTC’s tertiary mirror is an efficient ‘relay’ that can deflect light received from the secondary mirror to six different foci and therefore six different instruments.

The tertiary mirror will have a well-built bodyguard to protect and assist it. Like Rapunzel, the mirror will be locked away in a tower - but this tower will work with the mirror so that it can rotate in all directions.

There is something else too: the tertiary mirror will have to move out of the way, standing aside so that light can reach the Cassegrain focus just underneath the tower (see 3D animation). If they were playing chess, this movement would be like a ploy that results in a draw between secondary and tertiary mirrors: they will both win. Would you like to see it?

Solving problems

Light has to follow a precise path once it has been reflected by the primary and secondary mirrors. So that it does, telescopes have a tertiary mirror that ‘intercepts’ the light, deflecting it to the folded Cassegrain and Nasmyth foci.

The tertiary mirror’s design presented two problems:

Firstly, when the Cassegrain focus is being used it will have to receive light straight from the secondary mirror - and the tertiary mirror will be in the way. This is a slight problem. How can light from the secondary mirror reach the Cassegrain focus if the tertiary mirror is blocking its path?

Secondly, because light from the secondary mirror will also have to be deflected to the folded Cassegrain and Nasmyth foci (located in the wings of the telescope) there has to be a tertiary mirror. Without it we would see nothing!...So what can be done?

Some telescopes have a tertiary mirror that is removed and installed as and when it is needed. This takes time and effort and is not without risk to the mirror. To avoid removing and replacing the mirror, the GTC will use a design that solves the problem: a mirror with its own ‘lift’ that tilts automatically, slides out of the way and ‘parks’ itself. To make the mirror rotate, the tower - a structure 1.8 metres in diameter and 7 metres tall - will turn on its base, pointing the mirror at the focus to be used.

Natalia R. Zelman

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