Radio Telescopes
Anna Edmonds
For the last two issues we’ve been talking about telescopes that collect the light that we can see. However, there are things “out there” that we can’t see. They’re just as important a part of our universe as those we do see. Today we have a variety of tools to help us gain information about a wide range of the invisible electromagnetic radiation, including radio, X-ray, and gamma ray telescopes.
The first of these telescopes to be developed was that using radio waves. In 1932 Karl Jansky at Bell Telephone Laboratory was trying to find where the static in short-wave radio telecommunications was coming from. He realized that some of the signals he was getting were coming from out in space, and that these could be developed into a new way to study the stars. The signals were like the waves our radios receive. Using increasingly sophisticated techniques, scientists have become able to resolve those signals into data many times more complex than optical telescopes can with visible light. (BPAA member Jim Young was one of the Bell Labs scientists working in this field.)
Radio telescopes are like optical telescopes in that their curved mirrors, or disks, receive the signals and focus them to a point. Then they transfer the signals to a radio receiver rather than a viewer’s eye. The receiver then sends the data on to a computer where they are changed so that scientists can process and study them. These ’scopes look like a complicated TV disk.
In order to increase the power of the radio ’scopes, scientists have used the fact that radio waves can be sent through electrical wire to enable them to link two or more widely separated ’scopes. This technique is called “interferometry,” meaning the blending of electromagnetic radiation from a collection of ’scopes. The farther apart these ganged ’scopes are, the more detailed is the information they gather. With the help of very precise atomic clocks at each of the ’scopes, the data are combined after they are collected. One of these sets of ’scopes, known as the Very Long Baseline Array (VLBA) links twenty-five radio ’scopes located between New Hampshire and Hawaii. Now the hope is to add a radio ’scope on the Moon to the Earth-bound instruments.
There is one major, growing problem for radio ’scopes: They are a threatened species. Just as Jansky was studying troubling static in the 1930s, now the static of cell phones, garage door openers, police radios, and wireless computer networks use frequencies that can spill over into radio astronomy bands. This “pollution” interferes with the weak signals coming from space, at times leading astronomers to false information.
The interferometry of radio ’scopes has helped astronomers discover unexpected black holes, pulsars, and quasars. Radio ’scopes have probed back in time to almost the moment of the Big Bang; they have greatly increased our understanding of galaxies and the afterglow of gamma ray flashes.
In the next issue we’ll talk a little about other kinds of telescopes.