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How does a radio wave become a picture?

Published: Wed, 11/12/2019
WSRT

Planets, stars and nebula’s all emit radio waves, which are a form of invisible light waves. They are detectable with radio telescopes such as the dish telescopes at Westerbork and radio telescope LOFAR. But how do the researchers at ASTRON construct an image out of those invisible waves?

By Mischa Brendel

 

electromagnetic spectrum
Electromagnetic spectrum

All matter emits radio waves, but some matter does this at specific wavelengths. Hydrogen, for example, emits radio waves at 1420 MHz. Hydrogen also happens to be one of the most important building blocks in the universe. Stars are born from it. Therefore, measuring the amount of hydrogen in the universe teaches us a great deal about the formation of the universe. This makes radio waves one of the most invaluable resources for understanding our universe.

Radio waves are detectable with radio antennas. This can be done with the traditional dish antennas, like those at Westerbork. The 14 Westerbork parabolic dishes date back to the seventies, but are not as old as they seem: throughout the years the radio telescope has received several updates, of which the most recent in 2018. Each dish antenna carries at its center a receiver at which all the radio waves caught in the dish converge.

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Westerbork Synthesis Radio Telescope (WSRT) credit: Astron

Radio waves can also be detected with telescopes like LOFAR, which operates in a different way. This radio telescope basically is an assemblance of many telescopes spread out over several countries, in which each ‘telescope’ is a field full of radio antennas. The dish telescopes at Westerbork look at a single point in the universe simultaneously, a field full of LOFAR-antennas looks at a plane.

With LOFAR each antenna is its own receiver. Since the radio waves that are caught are very weak, radio telescopes have amplifiers to increase the signal strength. That signal is measured as interference. However, the amplifiers themselves also create interference. So how to measure the proper interference and ignore the rest? This is where complicated calculations and computations come in, explains D.Eng. Albert-Jan Boonstra, R&D program manager technical research at ASTRON. “The signal coming from space has a specific interference, while each radio telescope causes its own, unique interference. So, the interference which each telescope receives that is identical to the interference received by other telescopes must be the interference from space. We filter out the other interference.”

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Low Frequency Array (LOFAR), with the LOFAR core on the left and antennes for the low frequencies on the right. Credit: Astron

The radio waves also need to arrive at the receiver simultaneously. All time differences must be filtered out, because in order to receive a strong enough signal so that an object can be properly charted, the received radio waves are added up. Again, computational power is key.

By constantly taking measurements the telescopes produce tremendous amounts of data: for LOFAR this adds up to petabytes of data per day. Boonstra: “Ideally we’d store and process all of those data, but we cannot afford to do so. Therefore, we add up the signals from the antennas on a single field and only send through those accumulated data. This reduces the amount of data a hundredfold.”

A central system then calibrates the data: incorrect data are removed and corrected. Subsequently the data are correlated, which results in a correlation matrix. “Then it is up to the astronomers to make proper astronomic charts out of these data,” says Boonstra.

A lot of expertise comes into developing these radio telescopes. Not only in the field of astronomy, but also in the fields of antennas, electronics, algorithms and scientific computing. ASTRON cannot outsource the development of all these systems, many are too specific and push the boundaries of current technologies. Therefore, it develops these systems in-house. And that makes ASTRON much more than just a research institute.