The newest big thing in radio astronomy – Astronomy Magazine

This article originally appeared in the June 2014 issue of Astronomy.

Nearly 30 years ago, the worlds top radio telescope engineers and black-belt radio astronomers haggled over their requirements for an array of antennas that could investigate the deepest, darkest, and coldest places in the universe better than any other telescope ever made.

What they sought sounded like a starry-eyed wish list: 60 or more antennas able to survive blizzards and 100mph (160 km/h) winds yet also able move as fast as missile trackers. And thats not all. Their surfaces cannot deform more than a third the thickness of a human hair. Their electronics cant add noise to the data. Giant trucks must carry the antennas safely for miles across a high-altitude desert without dropping power to the cryogenic receivers. And the array wont work without a supercomputer that can perform 17 quadrillion operations every second.

Fast forward to 2014, and this seemingly fantastical telescope the Atacama Large Millimeter/submillimeter Array (ALMA) is complete. It is a leap in astronomical imaging akin to Galileo Galileis first use of a telescope, and similarly, its technology and early science have changed the business of astronomy forever. Achieving this marvel required the largest ground-based telescope partnership in history, an international collaboration between North America, Europe, East Asia, and Chile that collected $1.3 billion to design and build the worlds most complex astronomical instrument.

Engineering expectations

Radio telescopes gather light with wavelengths from fractions of a millimeter to hundreds of meters. Visible-light waves, by contrast, are only hundreds of nanometers long. Antenna size being equal, a radio telescopes ability to image the universe is to an optical telescopes capacity what finger-painting is to a color photograph.

To gather and focus enough radio waves to achieve similar or better resolution than their optical cousins, radio telescopes must be huge. Earths gravity limits the immensity of a single telescope, but ingenuity can counter that force.

The worlds most versatile radio telescopes are built as reconfigurable arrays of antennas, affording them maximum power and flexibility. Special-purpose supercomputers pair the data from each antenna with that from every other antenna across the array in some cases, up to thousands of miles away to create binocular images of the sky from many different perspectives. The farther apart two antennas are, the greater the resolution of their binocular vision. This groundbreaking technique is known as aperture synthesis and won a Nobel Prize for its pioneer, Sir Martin Ryle.

The resulting data provide often unequalled detail measurements that precisely reveal the spectra (emission of different wavelengths of light), shapes, positions, and distances of objects in space. ALMA, its 66 antennas spread a maximum distance of 9.9 miles (16 kilometers) apart, will have 10 times the resolution of the Hubble Space Telescope when the antennas are observing at their smallest wavelengths.

Unlike its shorter-wavelength cousins, such as Hubble, that collect light as energy packets that hit detectors and form pixels in an image, ALMA must process the light it collects as waves. Each ALMA antenna surface has been painstakingly hand-tuned to accurately reflect light waves as tiny as 400 micrometers long thats about the length a human hair grows in a day. If the dishes have bumps any larger than one-third the diameter of a human hair, then the cosmic waves are scattered away.

Also, submillimeter light waves crash into ALMAs receivers at frequencies as high as the terahertz range 1 trillion per second and no computer (yet) can handle a data stream like that. Therefore, all signals exiting ALMAs receivers have to be mixed with a longer carrier wave. A metronome-like device (called a local oscillator) sends this beat to each antenna.

To ensure these electronics do not introduce any signals of their own during the mix-down process (which electronics naturally do), engineers designed innovative, near-microscopic mixers that can be kept cryogenically cold. To reduce other noise, all eight receivers inside an ALMA antenna chill together in a giant thermos that contains 4-kelvin (452 Fahrenheit) liquid helium, which is bolted behind the dish. This technology has increased receiver sensitivity on Earth fourfold.

The antennas themselves are high-tech art in motion. Engineers from nearly every time zone on Earth came up with three different but equally elegant solutions to the ultimate 12-meter antenna wish list, and the array is an international family of these triplets. Although they look slightly different, ALMAs antennas all share the record-breaking capabilities that astronomers dreamed up 30 years ago.

Read more:

The newest big thing in radio astronomy - Astronomy Magazine