Saturday, June 28, 2008

Amateur Radio Astronomy: No-Cost and Low-Cost Approaches

A decent optical telescope costs at least $300 to $400. For free optical astronomy, all you need are your eyes. You can also do some radio astronomy for free—if you own an FM radio or a television set. We thank Tom Crowley of Atlanta Astronomy Club for many of the following ideas. Even they are affordable, don't drop it, because it breaks easily. If that's happen then I'll send you a nice condolence letter.
Have you ever witnessed a meteor shower? The streaks of light in the night sky can be quite spectacular.
Meteors are the bright trails of ionized atmosphere behind tiny bits of cometary debris that enter the earth’s atmosphere. Most meteorites are no larger than a pea. What if we told you that there was another way to watch a meteor shower—using a radio telescope otherwise known as an FM radio?
Meteor counts by radio are about ten times more accurate than visual observation—and, as with any radio observations, you can observe during the day or through clouds. It doesn’t have to be dark or clear outside. You may want to supplement your optical meteor gazing with your radio on cloudy nights.
Recall that the earth’s atmosphere is transparent to some forms of electromagnetic radiation and opaque to others. The upper atmosphere normally reflects low-frequency AM radio signals. In contrast, the atmosphere is transparent to higher frequency FM radio waves, which, as a consequence, have a shorter range. They usually penetrate and are not reflected by the atmosphere.
But something happens when a meteorite enters the atmosphere. Each piece of debris that tears into the atmosphere (at up to 40 miles per second), heats up the air around it and creates a tiny ionized (electrically charged) vapor trail in the upper atmosphere. These columns of charged particles can reflect even higher frequency FM radio waves. This temporary condition means that previously out-of-range FM broadcasts can (for a moment) be heard. During periods of known meteoric activity, stay at the low end of the FM dial and try to find FM radio stations that are from 400 to 1,300 miles away. You might call a distant friend to get the broadcast frequency of a few stations. When a distant station fades in for a second or two, you are indirectly observing a meteor. The trail behind it has momentarily reflected a distant radio signal into your receiver. It helps if you can hook up your radio to an outdoor antenna, but if the meteor shower is fairly intense, you should detect many events even without such an antenna. You can also try tuning your TV set to the lowest unused VHF channel. Again, when a distant station, normally out of range, fades in and the signal becomes strong for a second or two, you know a meteor has entered the atmosphere. (Note that this works only with a television receiving signals from an outdoor antenna—not via cable or satellite!)
Just by tuning in your radio, you can do some meaningful radio astronomy. You can make it more interesting by recording the events on tape, or keeping a written record of the number of events you detect per hour.
But amateur radio astronomy need not be limited to listening for distant FM radio or TV stations. If you are an amateur radio operator—a ham—you already have much of the equipment required for more serious radio astronomy. If you aren’t into amateur radio, you can get started for a highly variable but modest cost. The first step to take is to log onto SARA’s World Wide Web site ( for overview information.
Essentially, amateurs can use either nonimaging or imaging radio astronomy techniques. Nonimaging techniques (which monitor radio emissions without pinpointing locations) require a simple shortwave receiver, usually modified to receive a narrow band of frequencies, and a simple antenna system. With such equipment, you can track radio emission from Jupiter, solar flares, and meteor events. Imaging techniques (which provide more detailed information on the location and nature of the signal) require a more serious commitment of resources, including a much larger dish-type antenna, more sophisticated receiving equipment, reasonably elaborate recording equipment, and (probably) a rural location removed from most sources of radio interference. For purposes of this blog, we’ll restrict ourselves to the more approachable nonimaging techniques, which are more appropriate for beginners.

You Can Do This, Too!

Building a huge radio telescope like Arecibo or the Green Bank Telescope (GBT) takes a great deal of money, and so does operating one. Even if you had the cash, your neighbors (not to mention the local zoning board), might frown on your building even a modest 30-foot-diameter dish antenna in your backyard. However, remember that radio astronomy originated with non-astronomers, and there is still plenty of room in the field for amateurs, including amateurs of modest means. You can see the sky just like an ancient Aztec astronomer. A small but committed group of enthusiasts have formed the Society of Amateur Radio Astronomers (SARA). Most books for budding astronomers don’t discuss amateur radio astronomy, though it is a fascinating and rewarding subject.

What Radio Astronomers “See”

Insomnia is a valuable affliction for optical astronomers, who need to make good use of the hours of darkness when the sun is on the other side of the earth. But as Karl Jansky discovered so many years ago, the sun is not a particularly bright radio source. In consequence, radio astronomers (and radio telescopes) can work night and day.
The VLA, for example, gathers data (or runs tests) 24 hours a day, 363 days a year. Not only is darkness not required, but you can even make radio observations through a cloud-filled sky. The senior author of this book even observed a distant star-forming region in the midst of a storm during which lightning struck near the VLA and disabled it for a few minutes.
As the Dutch astronomer Jan Oort realized after reading Reber’s work in the 1940s, radio waves opened new vistas into the Milky Way and beyond. Radio astronomers can observe objects whose visible light doesn’t reach the earth because of obscuration by interstellar dust or simply because they emit little or no visible light. The fantastic objects known as quasars, pulsars, and the regions around black holes—all of which we will encounter later in this book—are often faint or invisible optically, but do emit radio waves.
The spiral form of our own Galaxy was first mapped using the 21 cm radio spectral line from neutral (cold) hydrogen atoms, and the discovery of complex molecules between the stars was made at radio frequencies.
The very center of our own Milky Way Galaxy is hidden from optical probing, so that most of what we know of our galactic center has come from infrared and radio observation. Since radio interferometers are detecting an interference pattern, radio data has to be processed in ways different from optical data. But the end result is either a radio image, showing the brightness of the source on the sky, or a radio spectrum, showing a spectral line or lines.

Monday, June 16, 2008

Interference Can Be a Good Thing

There is a way to overcome the low angular resolution due to the size of radio waves:
link together a lot of smaller telescopes so they act like one giant telescope. A radio interferometer is a combination of two or more radio telescopes linked together electronically to form a kind of virtual dish, an array of antennas that acts like one gigantic antenna. It is as if we had small pieces of a very large optical telescope (imagine a giant mirror with a lot of its surface area punched out), so that while an interferometer has the resolution of a very large telescope, it does not have the surface area or sensitivity to faint sources of a truly gigantic telescope.
The National Radio Astronomy Observatory (NRAO) maintains and operates the Very Large Array (VLA) interferometer on a vast plain near Socorro, New Mexico, consisting of 27 large dishes arrayed on railroad tracks in a Y-shaped pattern. Each arm is 12.4 miles (20 km) long, and the largest distance between 2 of the antennas is 21.7 miles (35 km). As a result, the VLA has the resolving power of a radio telescope 21.7 miles across. The VLA recently celebrated its twentieth anniversary.
For radio astronomers who want something even larger than “very large,” there is Very Long Baseline Interferometry (VLBI), which can link radio telescopes in different parts of the world to achieve incredible angular resolutions better than a thousandth of an arcsecond (.001”). From its offices in Socorro, New Mexico, the NRAO also operates the VLBA (Very Long Baseline Array), which consists of 10 radio dishes scattered over the United States, from Mauna Kea, Hawaii, to St. Croix, U.S. Virgin Islands. In 1996, Japanese astronomers launched into Earth’s orbit a radio telescope to be used in conjunction with the ground based telescopes in order to achieve the resolution of a telescope larger than the earth itself.

Bigger Is Better: The Green Bank Telescope

In the case of radio telescopes, size really does matter. The resolution of a telescope depends not only on its diameter, but the wavelength of the detected radiation (the ratio of wavelength to telescope diameter determines the resolution). Radio waves are big (on the order of centimeters or meters), and the telescopes that detect them are correspondingly huge. Also, the radio signals that these instruments detect are very faint, and just as bigger optical telescope mirrors collect more light than smaller ones, bigger radio telescopes collect more radio waves and image fainter radio signals than smaller ones.
Collecting radio signals is just part of the task, however. You may recall that, for practical purposes, very good optical telescopes located on the earth’s surface can resolve celestial objects to 1” (1 arcsecond—1 ⁄60 of 1 arcminute, which, in turn, is 1 ⁄60 of 1 degree). The best angular resolution that a very large single-dish radio telescope can achieve is about 10 times coarser than this, about 10”, and this, coarse as it is, is possible only with the very largest single dish radio telescopes in the world. The National Radio Astronomy Observatory has just commissioned the world’s largest fully steerable radio telescope. The 100 m dish will have a best resolution of 14”.
The world’s largest nonsteerable single-dish radio telescope was built in 1963 at Arecibo, Puerto Rico, and has a dish, 300 meters (984 feet) in diameter sunk into a natural valley. While its great size makes this the most sensitive radio telescope, the primary surface is nonsteerable—totally immobile—and, therefore, is limited to observing objects that happen to pass roughly overhead (within 20 degrees of zenith) as the earth rotates.

Anatomy of a Radio Telescope

The basic anatomy of a radio telescope hasn’t changed all that much from Reber’s dish—though the instruments have become much larger and the electronics more sophisticated. A radio telescope works just like an optical telescope. It is just a “bucket” that collects radio frequency waves, and focuses them on a detector. A large metal dish—like a giant TV satellite dish—is supported on a moveable mount (either equatorial or altazimuth). A detector, called a receiver horn, is mounted on legs above the dish (prime focus) or below the surface of the dish (Cassegrain focus). The telescope is pointed toward the radio source, and its huge dish collects the radio waves and focuses them on the receiver, which amplifies the signal and sends it to a computer. Since the radio spectrum is so broad, astronomers have to decide which portion of the radio spectrum they will observe. Different receivers are used for observations at different frequencies. Receivers are either swapped in and out, or (more typically) the radio signal is directed to the correct receiver by moving a secondary reflecting surface (like the secondary mirror in an optical telescope).

A Telephone Man Tunes In

The first true radio astronomer was not trained as an astronomer at all. Even to this day, many astronomers who work in the radio regime were trained as physicists and electrical engineers, and later learned to apply their knowledge to astronomy. Karl Jansky, the son of a Czech immigrant who settled in Oklahoma (where Karl was born in 1905), took a degree in physics at the University of Wisconsin. After graduating, Karl went to work in 1928 not as an astronomer, but as a telephone engineer with Bell Labs. The phone company was looking for ways to make telephone communications possible with shortwave radio, but the transmissions were bedeviled by all sorts of interference.
Now most people hadn’t given much thought to radio static. After all, static was something to be avoided if possible—meaningless noise that only interfered with communications. Jansky was given the assignment of studying sources of static at a wavelength of 14.6 m in an effort to track down the precise sources of radio interference and eliminate them.
On a farm in Holmdel, New Jersey, not far from Bell Labs, Jansky set up a very ungainly looking device, which he called a merry-go-round. It was a large directional antenna, which looked rather like the biplane wing of the Wright brothers’ first airplane. It was mounted on some discarded Model T Ford wheels and could be rotated through 360 degrees by means of a motor. Using this contraption, Jansky was soon able to identify all the known sources of radio interference except one. Jansky tracked the stubborn and mysterious interference. When amplified and sent to a speaker, the interference sounded like a faint hiss. The source seemed to be in the sky, since Jansky could track it rising and setting with the stars. But it wasn’t coming from just anywhere in the sky. By the spring of 1932, Jansky traced the primary source of radio noise to the direction of the constellation Sagittarius, which astronomers Harlow Shapley and Jan H. Oort had identified (from the distribution of globular clusters in the Galaxy) as the direction of the center of the Milky Way Galaxy. Using his merry-go-round antenna, Jansky had “discovered” the center of the much bigger merry-go-round that is our galaxy. There were other sources of radio noise in the sky as well, but Jansky noted that the sun itself was not an impressive source of radio noise. This observation was a bit surprising, since the sun is so close to us. He concluded that whatever the source of radio noise, it probably wasn’t distant stars.
Jansky published his “discovery” late in 1932, and the detection of radio signals from space appeared in national newspapers by the following year. Strangely enough, Jansky himself didn’t pursue the science he had accidentally created. As for most professional astronomers, they continued to look through only one of their two windows, the portion of the spectrum available to optical telescopes. It took another nonastronomer, Grote Reber, to appreciate the possibilities of what Jansky had discovered. In today’s image-conscious world, we might call Reber a nerd. But as the example of Bill Gates has shown us, some nerds go on to change the world. Born in Wheaton, Illinois in 1911, he grew up tinkering with radio transmitters, building one powerful enough to communicate with other ham radio operators all over the world. Like many early radio astronomers, he became an electrical engineer, but never lost his interest in amateur radio, and when he read about Jansky’s discovery, he tried, without success at first, to adapt his own shortwave receiver to pick up interstellar radio waves with wavelengths of 10 cm.
He tinkered with the electronics (trying longer wavelengths), and, in 1937, built a paraboloidal antenna 30 feet in diameter. With this, Reber not only confirmed Jansky’s discovery of radio waves from the direction of Sagittarius, but found other sources in the direction of the constellations Cygnus, Cassiopeia, and elsewhere.
Reber confirmed that the radio signals did not coincide with the positions of visible stars. Directing his dish toward such bright stars as Sirius, Vega, or Rigel, he detected nothing. But looking toward a starless area in Cassiopeia, he picked up strong radio waves. He had unknowingly detected a supernova remnant known as Cassiopeia A.

Dark Doesn’t Mean You Can’t See

On a clear night far from urban light pollution, the sky is indeed dazzling. Just remember that the electromagnetic information your eyes are taking in, wondrous as it is, comes from a very thin slice of the entire spectrum. As we mentioned in the last chapter, the earth’s atmosphere screens out much of the electromagnetic radiation that comes from space. It allows only visible light and a bit of infrared and ultraviolet radiation to pass through a so-called optical window and a broad portion of the radio spectrum to pass through a radio window.
Two windows.
If your house had two windows, would you look through only one?

Seeing in the Dark

“What’s an astronomer, Daddy?”
Spending much time around a little boy or girl can be pretty exhausting. All those questions! At least this one has a quick answer: “An astronomer is a person who looks at the sky through a telescope.”
“But, Daddy, the visible spectrum is squeezed between 400 and 700 nm. What about the rest of the electromagnetic spectrum?”
Smart kid.
Until well into the twentieth century, astronomers had no way to “see” most of the nonvisible electromagnetic radiation that reached Earth from the universe. Then along came radio astronomy, which got its accidental start in 1931–1932 and was cranking into high gear by the end of the 1950s. Over the past 40 years or so, much of our current knowledge of the universe has come about through radio observations. Radio astronomy is simply the study of the universe at radio wavelengths. Astronomers used to categorize themselves by the wavelength of the observations that they made: radio astronomer versus optical astronomer. Increasingly, though, astronomers define their work more by what they study (pulsars, star formation, galactic evolution) than by what wavelength they use. The reason for this change is that, in recent years, new instruments have opened the electromagnetic spectrum to an unprecedented degree. Astronomers now have the ability to ask questions that can be answered with observations at many different wavelengths.