Last week I wrote about optical ground based telescopes. In this post we learned about the electromagnetic spectrum and how visible light makes up only part of the spectrum. We also learned that all parts of the spectrum are useful in different ways for astronomy. In this article I am going to focus on the telescopes that are used to explore the universe in the radio part of the spectrum.
When it comes to seeing light from other wavelengths, such as radio, the type of telescope used changes. Radio telescopes often don’t look like telescopes at all. They can come in various shapes and sizes. In fact it’s probably best to describe them as antennas rather than telescopes. Some look like the satellite dish your TV reception comes in on while some look like a TV aerial. Radio astronomy began in 1932. An engineer called Karl Guthe Jansky was working for Bell Telephone Laboratories searching for sources of static that would interfere with radio telephony signals. He detected a faint hiss and found that it recurred every 23hr 56mins which is the time it takes for an object in the sky to return to the same point. He eventually worked out the source of the emission was from the centre of the Milky Way galaxy.
One of the advantages of radio astronomy is that cloud doesn’t matter and neither does daylight. This means that locations that are prone to bad weather would still make a suitable location for a radio telescope. One of the main concerns when picking a location for a radio telescope is interference from radio signals such as mobile phone cell towers.
The FAST radio telescope in China is currently the world’s largest radio telescope. It is a giant 500 metre dish built into a depression of a mountain. The disk focuses the radio waves to a collection of instruments handing above the dish.
Interferometry is when the signal from multiple telescopes point at the same object and the data is combined to give greater resolution than one on its own. We mentioned this in the previous article about optical telescopes. Interferometry is much more common in radio astronomy as it is much easier to line up the radio signals from different telescopes after the data has been taken. This has led to the development of telescope “arrays”. This is when multiple antennas/ telescopes are built to work together. One example of this is MeerKAT. It is located in South Africa and consists of 64 dishes.
Sometimes radio arrays are spread across huge distances. Indeed Ireland is part of a network of radio telescopes called the Lo Frequency ARray LOFAR. There are 14 sites spread across 6 countries including a receiving station at Birr castle in Co.Offaly.
The uses for radio astronomy are immense. Many objects emit light in the radio part of the spectrum. Objects like pulsars (fast rotating stars) have been discovered using radio astronomy. Other uses include mapping out the structure of galaxies and even listening for signals from other star systems that could be produced by intelligent life. The cosmic microwave background is the left over signal from the big bang and it was discovered using radio astronomy. Indeed the next time you hear static on your radio remember that some of that static comes from astronomical objects such as the sun and cosmic microwave background.
Paul Smith 