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A dome on a grand scale

13/01/2004

Building telescope domes is a costly business and at the moment there is a tendency to limit their size, but the GTC, which is at the Observatorio del Roque de los Muchachos on the island of La Palma, rears proudly up to a height of 26 metres and stands taller than any other telescope.

We have all had the experience of taking a special photograph only to have our hopes dashed when it comes out blurred. Distorting images is the bread and butter of photographers, whether amateur or professional, but things are different when it comes to astronomical research. Observation of an event that occurs only once every thousand years cannot be allowed to fail because of problems with capturing the image.

That is why ever-more complicated domes are being built, to cut out air turbulence and safeguard the quality and clarity of night-time observations.

A telescope is not like an ordinary camera. It is a much more complicated instrument that grasps radiation coming from outside the earth’s atmosphere. Telescopes use systems of lenses or mirrors to magnify the image of faraway objects, making them larger and clearer so that they can be seen and studied.

In recent decades enormous strides have been made in the technology for building telescopes. Today’s telescopes are too complicated and finely calibrated to be left unprotected against sunlight, heat and hostile atmospheric conditions. Costly and fragile equipment like this cannot be kept in an unfavourable environment, so some kind of shelter has to be built into the telescope.

THE DOME PROTECTS

Unlike other types, most optical and infrared telescopes are permanently protected inside a dome, which is kept shut during the day and opened at night to observe.

The GTC’s design incorporates several innovative technologies - one that puts it ahead of its predecessors is its dome. The dome is like an enormous jacket that protects the telescope from windy and humid conditions, facilitates ventilation and cuts the internal and external air turbulence that can compromise the image.

The GTC’s dome is a steel structure that rotates. It looks like a round helmet, is 34m in diameter on the outside, 26 metres high - as high as an 8 storey building - and weighs 500 tonnes. None of this will get in the way of the dome rotating, which it will do on the rail that it rests on at its base. The dome has a 13 metre wide slit, to be fitted with two mobile shutters that will slide out of the way when the telescope is observing.

NATURAL VENTILATION

Any difference between the air temperature inside and outside the dome can produce pockets of hot air which move into the telescope’s line of view. In other words they can affect the image, much as images seem to tremble when you look at a road surface in the distance on a hot day. To avoid differences in temperature the dome will be insulated to reduce heating by the sun during the day. It will also be air conditioned so that the daytime temperature inside the telescope chamber is kept the same as outside at nightfall.

Wind can also distort the image by making the telescope structure vibrate, but the GTC, where nothing is left to chance, will be fitted with a windscreen that will act like a venetian blind to minimise the risk of air turbulence.

In addition, an insulation chamber will isolate the base of the enclosure from the telescope to reduce the amount of heat exchanged between the two.

The GTC’s dome will have apertures to keep the temperature uniform throughout the telescope and to let air in from outside at night.

There will be two rows of 8 ventilation apertures, all covered by sliding doors (16 in total, each 4 x 4 m, 1500 kg in weight and shaped like a trapezium), plus a third row of apertures built into the base of the concrete cylinder, bringing the area dedicated to ventilation up to a total of 228 square metres.

The sliding doors will be used to open and close the apertures so that the atmosphere inside and outside the telescope chamber can be equalised. This will prevent the temperature in some parts of the telescope’s observation area being different to others, as this would affect image quality.

That is how conditions on the inside will be kept at their best, a key element in the overall plan to make observation from such a big telescope absolutely crystal clear.

Text: Ángeles Bravo

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