The story of a perfect paraboloid

2009/10/01 Roa Zubia, Guillermo - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Teknologia
• Astrofisika
• Astronomia

Since the lower structure was finished, it was almost a year until they made the plate of the Great Millimeter Telescope. Of course, building a large telescope at 4,600 meters in height is not easy. But it's not just that. To achieve the required precision, one of the great challenges of the telescope, located in the Orizaba National Park of Mexico, is the construction and maintenance of the plate.

The main reflector is a 50 metre diameter parabolic antenna. It is the largest built to collect millimeter waves from space. "The second largest is in Japan, at the Nobeyama observatory," says Itziar Aretxaga. "It's a 45 meter telescope, but it's not as good as ours. Transparency is not so good in Japan and the telescope is not so high, so water vapor absorbs very much millimeter waves."

Being the largest has advantages, because the big and the concrete are synonymous. It should be taken into account that most radiations from space are very weak and many of them are absorbed by the atmosphere. But the greater the plate, the more rays are received and the finer the instrument. This telescope allows to analyze those that so far have not been able to study.

However, its high difficulty from the constructive and maintenance point of view. It is necessary to collect as many rays as possible, and for this, in addition to the large telescope plate, it must have the most accurate parabolic form possible.

The main reflector (the plates) reflects the rays that receive a second reflector located in the center of the antenna. And this reflects them to where the sensors are. The more accurate the parabolic shape of the plate, the more rays will reach the second reflector.

Weight and heat

And there is the challenge in the case of the Large Millimeter Telescope. It is so large that it easily loses the perfect shape. It is not a deformation that is seen at first glance, but it is very evident in the results of astronomers; a certain deformation of the antenna prevents many rays.

In fact, if the main reflector is more than 70 micrometers away from the perfect shape (average in the entire antenna), astronomers do not give the telescope's performance good. Very low deviation. "The precision for the thickness of a hair is in a paraboloid with a surface of half football field", explains Aretxaga to give a comprehensible magnitude.

It should not be filled in each and every point of the antenna, but it is an average deviation of the antenna which, however, is very small. Deformation is simple and usually gravity is the main cause of deformation. When moving the antenna the paraboloid form is easily lost.

The problem has a solution. The antenna is not a single solid, but is made up of small sheets. 180 plates arranged in 5 rings (the interior is 12 plates, the second of 24 and the other 3 of 48). They are trapezoidal shaped foils that can be moved individually and adjusted to the form of paraboloid. A network of computers is constantly calculating the movements that need the plates to fit in the form of paraboloid.

In addition to gravity, thermal changes can be a problem for the telescope. For example, the direct or not impact of the sun is very important. Precisely for this reason, radiotelescopes are white and very bright, so that the skin reflects as much light as possible. But in the case of the Large Millimeter Telescope, only the painting does not solve the problem. It is too big for it. It needs thermal sensors dispersed by the antenna to receive the information of the temperature and transmit it to the computers for adjustments.

Against the wind

Even giving the perfect shape to the antenna, we must overcome other technological challenges. The most important thing is to face the wind, since the antenna acts as a candle of a boat. The wind moves the entire structure and the consequence is that there may be serious problems placing the telescope at a given point.

The wind problem has a difficult solution. In fact, the structure of the telescope is rigid enough to function with a wind of 10 meters per second. And engineers say that even when there is constant wind they can work with the shape of the corrected antenna. However, a continuous analysis will lead to better functioning. Little by little the engineers are learning, so they say it is a telescope that improves with time.