Turning on the Atacama Large Millimetre Array

Ken Tapping's weekly astronomy column from Dominion Radio Astrophysical Observatory


t the beginning of this month, an important new astronomical instrument began operating. It is the Atacama Large Millimetre Array, or ALMA: a large array of radio telescopes combined to form a powerful imager.  When fully completed, it will consist of about 50 movable 12-m diameter dishes.

Its main purpose is to image the cosmos at very short radio wavelengths, close to the infrared part of the spectrum. These wavelengths are important for penetrating the birth clouds of stars and new planetary systems to see exactly what is going on inside, and to observe the very young universe. Of course it will be useful for many other research topics in astronomy.

ALMA consists of a large number of 12-m dishes scattered over about 16 km. Instruments like this are too complex and expensive for any single nation to develop alone. ALMA is a partnership between nations all over the world, including Canada. Important pieces of ALMA technology were designed and constructed at our institute, and Canadian scientists are at the telescope, and helped bring it into operation.

The technical complexity and cost are only two of the issues that had to be faced. The short wavelengths that ALMA was developed to observe are severely affected by the atmosphere, particularly water vapour. So to be effective, the instrument would have to be located at a high, dry, site.  We would like also for that site to provide a good view of the core of our galaxy, the Milky Way. Unfortunately, that means we couldn’t put ALMA in the Northern Hemisphere, where the centre of the Milky Way only peeks shyly at us close to the southern horizon. We needed to put ALMA in the Southern Hemisphere, where the galactic core passes almost overhead. The best site turned out to be a high, dry plateau in Chile.

The Atacama Plateau is one of the driest places on Earth, and is also at a high altitude. This plateau has been used to emulate for the Martian surface when developing technology for space missions to Mars.  ALMA is located about five kilometres above sea level, and above most of the atmosphere, under very low humidity. At these altitudes, people’s ability to do heavy work is severely limited, and visitors have to be in good health. Therefore to the greatest extent possible, the major tasks are done at an Operations Support Facility, which is lower down, about three kilometres above sea level, where it is easier to work.

ALMA was not an easy instrument to build. Firstly it required new developments in antenna and receiver technology, and a significant investment in new computing techniques. Then there was the problem of erecting buildings and hitech instruments at a very difficult site. However, the only alternative would have been to put the telescope in orbit, which is currently impossible, or putting it on the Moon. That would be a really good site, but it will take a few decades before such an instrument becomes even remotely possible. Our new machine on the Atacama Plateau should keep us busy doing new science and technical development until then.

Jupiter rises around 7 p.m, Mars comes up around 2 a.m. The Moon will be new on the 26th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.