THE NIGHT SKY HAS CAPTIVATED OUR IMAGINATION for thousands of years. Indeed, the major civilizations of human history, from the Egyptians to the Maya, have partaken in extensive and independent studies of the stars and our place among them. The Maya, for example, developed a calendar based on their general knowledge of the many periodic celestial events visible to the naked eye. Their accomplishments are remarkable in spite of the limited technology of their time. By contrast, today we have photographed some of the farthest galaxies in our universe, sent probes to investigate other planets in our solar system and have even walked on the moon. While we may have advanced technologically far beyond our ancestors, we have come to know that the universe is just as mysterious and beautiful as it has ever been.
Serious astronomy began with the invention of the refracting telescope in the early 1600s. With it, Galileo observed the four most famous moons of Jupiter and our philosophies changed accordingly; we were not the center of the universe after all. The invention of more powerful telescopes since Galileo’s time has further shaped the way we understand the inner workings of the universe. Modern astronomers can now observe the universe with instruments that detect light not visible to the human eye, from low energy radio waves to high energy gamma radiation. The advantage is easy to understand: different types of light serve as indicators of the various underlying physics governing a system.
One of the most exciting aspects of astronomy is its accessibility. Amateur astronomers worldwide can observe the same objects in the sky as professionals, albeit without some of the precise gadgets permitted by millions of dollars in project funding. Though most amateur astronomers today use optical telescopes to explore the night sky, technology has advanced so much that amateurs can inexpensively “see” other parts of the electromagnetic spectrum, such as radio waves. A particularly simple radio telescope involves 100m of shielded copper wire, a simple-to-build preamplifier, and a computer with a sound card. It is capable of detecting radio waves from space, such as from the sun, and even encrypted U.S. military signals.
The idea is to wind the wire, such as #26MAG copper wire, into a large loop of approximately one to two meters in diameter around a stable frame of sorts – perhaps a keyboard stand. The smaller the loop, the more turns of wire are needed in order to increase sensitivity; conversely, the larger the loop, fewer turns of wire are necessary. As far as antennas go, this design is surprisingly capable and allows for a useful signal to be detected anywhere between 5 and 48 kHz. How does it work? The large loop effectively behaves like an inductor-capacitor circuit, picking out electromagnetic radiation passing through and around it. The two ends of the antenna feed into the preamplifier for some basic signal processing.
The purpose of the preamplifier (preamp) is twofold. First, it amplifies the weak signal received by the telescope so that it can be seen on the computer later; and second, it must not add additional noise into the system that can potentially drown out a meaningful signal. An excellent receiver for the job is the SuperSID preamp, which can be obtained from the Stanford Solar Center website or can be built by the electronically brave at home with the necessary parts. The signal from the antenna passes through the preamp, gets amplified, and is sent to the computer via the Line In jack in the audio card for final processing. Check out the block diagram below for clarity.
Spectrum Lab, a free software, can take the processed signal from the preamp and separate it into its constituent frequencies. What does that mean? The real signal received is a superposition of countless radio waves at a range of frequencies. Spectrum Lab quickly decomposes the signal in terms of frequency space so that the strength of the signal at particular frequencies can be easily seen.
A sample output from a real signal in Spectrum Lab can be seen above. The jagged line at the top of the figure is what the signal looks like at a particular instant in time and the numbers below it indicate the various frequencies the signal contains in Hertz. Sharp peaks can be seen at 21.4kHz and 24kHz, indicating that the antenna detects more radio waves at these frequencies than the others. The history of the signal in time is vertically shown in the colored portion of the diagram. Dark blue portions represent weak signal, while the brighter, orange-yellow portions indicate strong signal. The sharp peaks in the signal are no coincidence since the vertical lines present at the 21.4kHz and 24kHz imply that the strength of the signal there is constant in time. Other things can be observed too. For example, broadband emission from the wind moving the telescope, or perhaps a nearby microwave can be seen as the long horizontal yellow lines. If one looks more closely, another, weaker signal is visible at about 19.8kHz. All the frequencies pointed out so far correspond to strong emissions from U.S. military radio towers.
Of course, radio astronomy is not limited to long wire antennas alone. Radio astronomers can use dishes, similar to satellite tv dishes, to focus radio waves into a detector for analysis. In this way, the very first neutron stars were discovered. The famous Cosmic Microwave Background was detected by accident using similar ideas. Radio astronomy is an exciting avenue for amateur astronomers who want to view the universe through an unfamiliar lens, while learning a lot about electronics and physics along the way. Through this medium, many amateur astronomers have carried on the tradition of astronomy started by our ancestors, and it is exciting to think about doing the same in the future.
Andrei holds an undergraduate degree in Honors Mathematical Physics and will attend the Perimeter Institute for Theoretical Physics starting in August. His interests cover computer science, consumer electronics, engineering, mathematics and physics.