Radio Astronomy

Nowadays, most astronomical work is done with cameras and computers. It is probably not surprising that astronomical cameras are more sensitive to light than the human eye. Your eyes might tell you "that galaxy kind of looks bright in the centre, and maybe has a spiral structure.” A camera will tell you exactly how bright every part of the image is, and how it changes over time. Astronomers can learn a lot from this highly precise data.

An astronomical camera can also have an attachment called a spectrograph that works like a prism. The spectrograph allows the camera to pick up small changes in colour. So why does colour matter, aside from making pictures of objects in space look pretty? Well, different colours represent different wavelengths of light.

If you think of a wave, you might imagine stretching it to make gradual crests, or compressing it to make very sharp crests. The wavelength of a wave is the distance from crest to crest. We see the colours of a rainbow go from red to violet, but that isn't the entire picture. That is just the picture we can see with our eyes. Red light has longer wavelengths than violet light. Light can have wavelengths much, much longer than red, and much, much shorter than violet. We can only see a tiny fraction of all light. We call the entire possible range of light the electromagnetic spectrum (EMS).

We give different names to the different ranges of radiation. From the longest to the shortest wavelength they are: radio waves, microwaves, infrared, visible light, ultraviolet light, x-rays, and gamma rays. The visible part of the spectrum is actually very small. It is the part that people are most familiar with because it is the part that we see. So what does this have to do with astronomy? 

Everything. Just because humans can only see visible light, that doesn't mean that everything in the universe emits visible light. There are events and objects that can only be detected in different parts of the EMS. Some details of visible objects, like the Sun, can only be detected when viewed at these other wavelengths. So what do astronomers do to see these non-visible things? They build telescopes that can see these wavelengths.

Radio Astronomy

Radio astronomy is a field of astronomy that studies objects in space using radio frequencies. This means we can use radio waves instead of visible light to see stars and planets. 

The discovery of radio waves, like so many great discoveries in science, happened by chance. Karl Jansky was an engineer with Bell Telephone Laboratories. In 1931, he was investigating static that might interfere with radio-telephone service. He was using a large, rotating antenna which he built himself! 

He recorded signals from all directions for several months. He noticed an unknown type of static which repeated every 23 hours and 56 minutes. He discussed this puzzling static with a friend who was an astronomer. His friend pointed out that the time between the peaks of the strange static was the same as that of a sidereal day. That is the time it takes for an object in the sky to come back to the same location after one rotation of the Earth. This led Jansky to think that his strange static came from beyond the Earth. Jansky looked at astronomical maps and compared them to his observations. Astronomical maps are maps which show where celestial objects are located. He concluded that the static was coming from the Milky Way Galaxy and in the direction of the constellation Sagittarius. Not long after, an amateur radio operator named Grote Reber built the first dish-shaped radio telescope in his backyard in Illinois in 1937. Reber was one of the pioneers of what became known as radio astronomy. 

Reber and others found that since radio waves have a wide range of wavelengths (3 m to 30 m), radio telescopes need to come in different shapes and sizes. You may have seen these giant dish-shaped antennae in the movies (A).

More recently, radio observatories have been able to connect (network) different dishes together. The dishes can then operate as if they were one giant dish. This process is called interferometry. The Very Large Array (VLA) in New Mexico (B) is a good example of interferometry. Their array consists of 27 - 25 m diameter radio antennas set up in a Y-shaped pattern.

How do Radio Telescopes Work?

Radio telescopes and optical telescopes work differently. However, they do have some features in common with reflecting telescopes, though.

The big bowl-shaped part of a radio telescope is called the dish. The dish is a giant parabolic (concave-shaped) reflector. The dish takes incoming radio waves and brings them to a focus, like a mirror in a reflecting telescope. The telescope in the diagram below has a Cassegrain design. 

This type of telescope uses a secondary reflector, like the secondary mirror in a reflecting telescope. It also has a feed horn to bring the radio waves to a sharp focus. At the feed horn is an antenna. The antenna converts the radio waves into an electric current. This happens because the radio waves cause electrons to move in the antenna. The electronics in the radio telescope are often cooled with liquid nitrogen or liquid helium. This helps to reduce random electrical currents, or noise. The less noise there is, the easier it is to detect weak signals.

The other main parts are the tuner, the amplifier and the computer system. The tuner is like the dial on a radio. It allows the astronomer to focus on a single radio signal from the thousands that come into the antenna. Telescopes used by SETI are set up so that they can listen to more than one radio signal at a time. The amplifier is a device that increases the strength of the weak electrical current caused by an incoming radio signal. Finally, radio telescopes are hooked up to computer systems. These are used to record and store the electronic signals, analyze data and control the telescope's movements.

Telescopes for Other Wavelengths 

There are other telescopes that astronomers use to study the Universe using other wavelengths. Microwave telescopes have given us insight into the cosmic microwave background, a radiation field that permeates the entire universe, created shortly after the Big Bang. Infrared telescopes are good for peering through the dust in our galaxy that blocks visible light, allowing us to see things that might otherwise have remained invisible. Ultraviolet telescopes have given us insight into the chemical composition of our galaxy, which helps us understand how it changes over time. X-ray and gamma ray telescopes can detect high energy particles from objects such as neutron stars and black holes, giving us plenty of data with which to study these objects.