Effectively a monster: the SKAO

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The thing is, it won’t have a dish that large.

The word to remember here is “effectively”. The telescope will consist of several thousand small antennae, distributed thousands of kilometres apart. In fact, there’s even an entire ocean that will separate some of them. If you’re puzzled, remember that radio telescopes are different from optical ones in this sense: the signals from each of those antennae can be combined so that they, effectively, act as one gigantic telescope.

In fact, this is actually the way most radio telescopes in the world are built. The Giant Metrewave Radio Telescope (GMRT) between Nashik and Pune, the Owens Valley Radio Observatory (OVRO) in California, the Australia Telescope Compact Array (ATCA), the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands, the Very Large Array (VLA) in New Mexico, and more—all of these are actually arrays of antennae.

The VLA has 27 different antennae, each 25m across. The GMRT has 30, each 45m across. Each of the GMRT’s antennae are fixed to the ground, though the dishes can be tilted and rotated to point in different directions. The 30 dishes form an approximate “Y” layout, with a maximum separation between individual dishes of about 25km. The VLA’s smaller dishes form a Y-shape too. Mounted on railroad tracks, they can be moved along the arms of the Y so that each arm is up to 21km long: a slightly larger array than the GMRT.

The point of these arrays is simple: working together, they simulate much bigger telescopes.

To understand this, imagine that you, me, and our mutual friend Shormishtha go to a concert by a large orchestra. Unfortunately, we delayed buying tickets till the last minute, so we have three widely-separated seats. I’m near the string section, you’re close to the percussionists and Shormishtha is lucky enough to be close to the singer. Because of our respective proximities, the concert will sound different to each of us. But suppose we recorded the concert. By combining our three recordings, we can produce a soundtrack that’s different from each of our individual efforts, but closer to the full sound of the orchestra.

All three of us might have excellent hearing and recording devices, and all might report that it was a fabulous concert. But the three of us, sitting far apart, effectively embody a much larger ear than our individual aural organs. That joint recording we produce, then, is a more accurate rendition of the orchestra’s performance than whatever each of us heard.

This is analogous to what happens with the GMRT, for example. Each of its 30 antennae is an instrument sensitive enough to “listen” to radio transmissions that criss-cross the universe. Thus each on its own is valuable. But combine the signals each of them receive—and remember that the 30 are also sitting far apart from each other—and we get a much more accurate, detailed picture of whatever the GMRT is listening to.

Working like this for nearly 30 years now, the GMRT has produced some, well, stellar results. For example, in 2018, astronomers used it to discover an object called J1530+1049, the furthest-known radio galaxy, about 12 billion light years away. (Radio galaxies emit almost no visible light, so they are detectable only by radio telescopes.) Other radio telescope arrays have their own stellar results too.

The point here: A single antenna would not have been able to find J1530+1049. The GMRT did because it is an array of widely-separated antennae.

Which naturally raises the question: why not separate the antennae even further? After all, the universe is a vast place, a gigantic orchestra, with endless streams of transmissions to listen in on. What if we had ears distributed beyond–way beyond—a few dozen km from each other?

Well, that’s what the Square Kilometre Array Observatory (SKAO) project is meant to answer. Several countries will cooperate to build it. The antennae are being set up mostly in Australia and South Africa, with a few in other African countries like Botswana, Zambia and Madagascar. All those countries, because the southern hemisphere offers better views of our Milky Way galaxy. But also because with generally lower population densities in countries south of the Equator, there’s less interference from earthly radio sources to contend with.

The SKAO’s headquarters will be at the Jodrell Bank Observatory in the UK. That’s where signals from the SKA’s antennae—well over 130,000 of them!—will be combined. By doing that, the SKAO will effectively be one giant radio telescope, sensitive even to extremely weak signals—in fact, 50 times more sensitive than any other radio telescope on Earth. This also means it will be able to survey the sky up to 10,000 times faster than other radio telescopes.

From the start, India has been part of the SKAO project too. While there won’t be any of its antennae here, the plan has always been that India and the UK will process and analyze the vast amounts of data coming out of Jodrell Bank. Both countries are working to set up super-computing facilities to do this. In January this year, India formalized its participation by announcing that it will join the SKAO as a full member, and approving funding for the project. The SKAO’s director-general, Philip Diamond, commented: “Joining the SKAO is a natural step for a country with such a strong tradition of radio astronomy research.”

That strong tradition owes plenty to an authentic jewel of Indian science: the GMRT near Nashik.

Finally, you may wonder why I said above that the SKAO will have a dish covering one square km, and then that it won’t. Well, take the GMRT: 30 dishes, each 45m across. A little arithmetic tells us that’s a total of a little less than 0.05 square km. Similarly, the VLA covers about 0.013 sq km.

In contrast, the SKAO’s 130,000+ dishes will cover one full square kilometre. That itself tells us that its individual dishes will, on average, be far smaller than those in existing radio telescopes such as the VLA and GMRT. But on the whole, the SKAO is effectively over 20 times larger than the GMRT. A monster. I can’t wait for 2027, when it will start peering into space.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.



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