Loss of an icon

Last December, when the Arecibo radio telescope collapsed, radio astronomy lost an icon.

This instrument was regarded as a research tool for radio astronomy, but actually a nice, big dish antenna can be used for a variety of radio projects.

In short, for almost anything requiring collecting a lot of weak radio energy coming from something in the sky, such as radio astronomy, or detecting radio signals from distant spacecraft, or even extraterrestrial civilizations.

It was also used for interplanetary radar, which involves squirting an immense amount of radio energy in the direction of a distant planet or asteroid, and then detecting the incredibly weak signal echoed back.

The echoes could be used to measure the precise distance and movement of an object and even image it. Until spacecraft landed on the surface of Venus, the only way we had seen the surface of that permanently cloud-shrouded world was by radar. The antenna also had military applications.

However, for us it was a radio astronomy icon, a key instrument of its time.

Back in the 1960s when young, wanna-be radio astronomers were getting radio astronomy books from their local libraries — yes, those libraries did have books on radio astronomy — there were always pictures of two iconic radio telescopes, which were state-of-the-art at the time.

They were the 75-m diameter dish at Jodrell Bank, in the U.K., and the 305-m diameter antenna at Arecibo, Puerto Rico.

The Jodrell Bank instrument was mounted so that it could be pointed at any point in the sky, and was probably the largest radio telescope of that kind achievable at the time.

It was, and still is an engineering challenge to make a large dish that will stay precisely in shape as it moves around, and is subject to the wind and uneven temperatures across it. Something bigger would require a completely new approach.

That's when, in the early 1960s someone pointed out the potential of a large, saucer-shaped sinkhole, located near Arecibo, in Puerto Rico.

The idea was to mount a fixed dish in that depression. It could be solidly and precisely mounted because it was not intended to move. Of course, a fixed antenna like this has one huge shortcoming; it is permanently looking straight up.

However, the engineers got around this problem to some extent by making the dish with a spherical profile rather than a parabola, like almost all dish antennas, and by moving the signal collection point to different positions over the dish.

Collecting signals from a spherical antenna is not easy, because they are not fully focused, which is why everyone uses parabolic antennas.

On the other hand, the result was a dish with a diameter of 305 metres, easily the biggest in the world, capable of seeing a large patch of sky centred overhead.

There are still some radio telescopes being built that use large, fixed antennas.

The CHIME instrument at our observatory is an example, but by using modern signal processing techniques, it can see most of the sky over the observatory, without mechanically moving anything.

However, the mainstream these days is to make huge radio telescopes out of lots of small dishes. The Square Kilometre Array, now under construction in South Africa and Australia, will use thousands of them.

The big difference is that today we have the computing power needed to process all those signals. In the 1960s we did not.

Arecibo was a film star too. It was used in the movie Contact. It also played a role in the James Bond movie, Goldeneye. This unique instrument will not only be missed by scientists; it will also be missed by moviegoers.

A video of the collapse can be seen at https://www.youtube.com/watch?v=ssHkMWcGat4

• Mars is high in the south just after dark.
• The Moon will reach First Quarter on the 19th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.