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Skywatching

ALMA is a winner

Despite its austere beauty, the Atacama Plateau in Chile is one of the least hospitable places on Earth.

It is a 1,000-kilometre strip of land west of the Andes Mountains.The plateau is high enough for altitude sickness (basically the consequences of lack of oxygen) to be a hazard for anyone working there.

It is very dry; the average rainfall is maybe a few millimetres a year in some locations, while at others, rainfall has never been recorded. It is very, very dry. The terrain looks just like the surface of Mars, and has been used for testing Mars rovers and filming Mars movies.

There is even a valley called the Valley of Mars. It is also known as the Valley of Death. There are volcanoes and frequent earthquakes. So, of course, we chose the Atacama Plateau to be the site of one of the largest radio telescopes we have made so far.

The Atacama Large Millimetre Array, or ALMA, consists of 66 movable dish antennas forming an array about 16 km across. It was built and is operated under an international partnership including Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile.

The instrument is located at an altitude of about five km. At that height, the air is too thin for people to do precise work for long periods, so the workshops and other support facilities are located at a lower altitude, about 2.9 km above sea level.

The antennas are moved to the telescope site by huge, specially designed transporters. Installing the telescopes on-site or moving them to different locations, is precision work, and the transporter drivers have oxygen cylinders in their cabs to prevent altitude sickness affecting their concentration.

Considering all the challenges involved in building and operating such a large and technically complex instrument at such a location, we cannot help asking an obvious question:  why?

The universe is awash with electromagnetic waves. There are radio waves with lengths of metres or longer, which tell us about cosmic electrons and galactic magnetic fields, exploding stars and strange goings-on in the solar corona.

There are centimetre waves that tell us about quasars, black holes and the chemical processes going on in the clouds of gas and dust between the stars. Then there are millimetre waves, given off by asteroids, planets, comets and the clouds of gas and dust in the process of forming new planets and stars.

The millimetre emissions reveal an important chapter in the story of how we got to be here. The big problem is that millimetre radio waves are absorbed by our atmosphere, in particular, by water vapour.

One day we will be able to put big radio telescopes on the moon or in space, where these things won't be a problem. However, for the foreseeable future, our large-scale radio astronomical activities will be confined to the surface of the Earth.

We chose to build the radio telescope at the highest, driest place we could find, above a good fraction of the atmosphere and above most of the water vapour.

This instrument posed other technical challenges. High-sensitivity millimetre-wavelength radio receivers, previously built as one-offs, had to be produced in quantities, one set per antenna, and a custom-made signal processor had to be developed to deal with the tsunami of data coming out of the instrument.

ALMA has imaged planetary systems in the process of being born. In some cases it has revealed gaps in the disc of collapsing material, showing where new planets are growing.

It is imaging the material involved in forming stars and planets in galaxies other than our own, and it was one of the radio telescopes used to make the first ever image of a black hole.

  • Mars lies very low, getting lost in sunset glow as it approaches the other side of the sun.
  • Jupiter, shining like a searchlight, rises before midnight.
  • Saturn rises at 1 a.m.
  • The moon will be new on June 3.


More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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