GTCdigital
News
Comparisions
30/07/2003
Sizes and measurements are all well and good but they are often difficult to visualise without making comparisons: a wood the size of ten football pitches, a vehicle as heavy as a fully grown elephant, a bug as small as a pinhead...there are endless examples of how dimensions can be made more meaningful.
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Given everything that it needs to work properly and all the different parts that go into it, a telescope like the Gran Telescopio CANARIAS can be compared to lots of things. We thought this would make for a fun issue of the bulletin and it might give you a few surprises!
WEIGHTS AND MEASURES
For an idea of the GTC’s observing potential, think of a telescope that can see as well as 4 million human eyes. It would be able to make out the headlamps of a car 20,000 km away - that's the distance between Spain and Australia).
The telescope enclosure will be 41 m tall, just 6 metres short of the Statue of Liberty.
The telescope base will support the dome and will bear a total load of 500 tonnes, about the weight of a herd of 62 elephants.
The diameter of the GTC’s primary mirror will be around 10.4 m and it will be made up of 360 segments, each weighing 450 kg. In other words, each one will weigh as much as a prize bull.
However, despite their weight, each segment will be no more than 8 cm thick. Smaller diameter primary mirrors, like the 8.2 m mirror at the VLT in Cerro Paranal (Chile), can be up to 17.5 cm thick. If all the segments of the GTC mirror were placed one on top of the other their combined height would be no more than 3 m (288 cm).
The GTC mirrors are remarkable for the care that has been taken in their design and manufacture. The vitreous ceramic segments have been polished to within a margin of error of 15 nanometres, which is one three thousandth the width of a human hair (a nanometre is one thousandth of a micra, or 0.000001 mm). The entire mirror will weigh 16 tonnes, more or less the same as an adult male grey whale.
Great care must be taken when the mirrors are being manufactured, so that any irregularities are kept to less than 90 nanometres. If it were possible to build a mirror the size of Spain and Portugal, this would translate to any “mountains” being no more than 1mm high.
The base material for the mirrors is Zerodur, a substance similar to that used in cooker hobs. Its principal characteristic is its low expansion coefficient when subjected to high temperatures, which is vital in astronomic instruments. “Schott”, the German company that is making the mirrors, manufactures all kinds of glass objects: from telescope mirrors, television tubes and baby’s bottles to elegant, hardwearing and durable glassware.
Although the mirror will be segmented, each of the fragments will be just 3 mm apart.
The GTC will be a major triumph of engineering . The metal dome structure alone will be made up of some 59,000 parts: around 16,000 screws (4000 kg), 43,000 nuts (1500 kg) and about 450 kg of washers. That's a total of 6 tonnes just for these items.
Lastly, the most important feature: all 500 tonnes of the telescope, floating on a thin film of oil, will be capable of being moved with just a touch of the hand.
Now isn't that surprising?
Text: Rubén García Herrera
Natalia R Zelman