Dish of the day off the menu

THE Arecibo Radio Telescope, one of the world’ s most iconic astronomical instruments, will be used no more. It will be decommissioned and dismantled, following failures of two cables and fears of imminent collapse of the entire structure.

The telescope, managed by the University of Central Florida (UCF) for the United States National Science Foundation, has a 305m-diameter “dish’’ nestled in the hills of Puerto Rico.

It was completed in 1963 and has been used in very many astronomical endeavours. These have included its use as a radar dish that has shown the sizes and shapes of many Near Earth Asteroids (N E As) that have the potential to one day collide with our planet.

Among much other important research, it has been used to study pulsars, measure the rotation period of Mercury, and even send the famous“Arecibo Message ’’ in 1974, which announced our presence to any alien civilisations that may receive it.

A dish-type radio telescope works by focusing celestial radio waves, which are then picked up by a receiver placed above it. Its radar capability has led to locals giving it the name “ElRadar’’.

Most other famous radio telescopes, such as the one near Parkes in New South Wales, are steerable, meaning that the dish can be moved to aim it at different parts of the sky. However, the Arec ibo dish is too large to be moved.

To receive radio waves from different parts of the sky, the receiver itself was moved around, to collect radiation focused by the dish at different points.

Because of this design, the radiation could be collected only from a limited range of directions within an angle of about 20 degrees of overhead (the direction called the zenith ), but the coverage was good enough to do plenty of useful work.

The troubles began in August, when a steel cable slipped out of its socket. The cable was not one of the main supporting cables, but it damaged a receiver and tore a gash, about 30 ma cross, in the dish.

Views of the dish damage look quite dramatic, but even so, it was planned that the telescope would be repaired.

It was the subsequent failure of a main cable, on Nov em

ber 6, that was to signal a far more serious problem. It broke, and fell on to the dish, causing more damage.

It is suspected that this second failure was a result of the first: following August, the structure was weaker, and there was more load on the remaining cables.

Engineers investigating the situation came to the conclusion that the structure was in danger of collapse. Safety concerns dictated that it was inadvisable to attempt to save it. It will therefore undergo a “controlled decommissioning’’, with as many parts as possible saved for futurere-use in other instruments.

It is the end of an era. For so many astronomers today, the Arecibo telescope has been part of astronomy all of their lives, just as the internet has always been for people a generation or so younger than I.

Some years ago, a colleague and I made an official visit to the telescope, not to make radio observations but to present some of the ashes of noted radio astronomer Gro te Re ber.

Reber spent so much of his life in Tasmania, and his most famous work here involved his radio astronomy antenna array near Both well.

We walked around the circumference of the dish—I had not done that before — and were struck by the enormous size of the structure. It’s one thing to write down its diameter in metres, but to make the 1 km walk around it is another!

The Arecibo telescope has appeared in a few movies, the most memorable one for me being the James Bond movie Golden eye, which was made in 1995. It was a spectacular setting.

Radio astronomy research will, of course, continue around the globe, including here in Tasmania, where the University of Tasmania is actively involved in world-class work. However, we have lost an icon. Martin George is manager of the Launceston Planetarium(QVM AG ).