Radio telescope

Lore & Background

The rapid development of radar during World War II created technology applied to radio astronomy after the war, and radio astronomy became a branch of astronomy, with universities and research institutes constructing large radio telescopes.

Reader's Guide

Radio telescopes are essential for studying the radio frequency portion of the electromagnetic spectrum, complementing optical astronomy. They are typically large parabolic dish antennas, used individually or linked in arrays. Radio observatories are preferentially located far from major population centers to avoid electromagnetic interference from man-made devices. The range of frequencies in the radio spectrum is very large, so antenna designs vary widely: at longer wavelengths, directional arrays or large stationary reflectors with movable focal points are used; at shorter wavelengths, parabolic dishes predominate. The angular resolution of a dish is determined by the ratio of its diameter to the wavelength observed. signal. The increasing use of radio frequencies for communication makes astronomical observations more difficult, and negotiations to defend frequency allocation are coordinated by the Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science.

Did You Know?

• The Arecibo radio telescope, which collapsed on 1 December 2020, was one of the few radio telescopes also capable of active radar imaging of near-Earth objects.

From Static to Stars

The story of radio astronomy begins not with a quest for the cosmos but with a mundane engineering problem. In 1932, Karl Guthe Jansky, working at Bell Telephone Laboratories in Holmdel, New Jersey, was tasked with identifying sources of static that disrupted radiotelephone service. His solution was a rotating array of dipoles and reflectors tuned to 20.5 MHz, roughly 100 feet across and 20 feet tall, mounted on a turntable that let him sweep the sky in every direction. Colleagues nicknamed it "Jansky's merry-go-round." After months of logging signals from all directions, he sorted the interference into nearby thunderstorms, distant storms, and a faint, steady hiss of unknown origin. That hiss repeated on a 23-hour, 56-minute cycle—the sidereal day—telling him the source was fixed on the celestial sphere. Cross-referencing with optical star charts, Jansky traced the radiation to the Milky Way, strongest toward Sagittarius. Five years later, amateur radio operator Grote Reber built a 9-meter parabolic dish in his Wheaton, Illinois backyard, confirmed the galactic source, and carried out the first high-frequency sky survey, effectively founding the discipline.

Engineering Diversity Across the Spectrum

Because the radio portion of the electromagnetic spectrum spans an enormous range of wavelengths, no single antenna design can serve all observations. At the long-wavelength end—30 meters down to 3 meters, corresponding to 10 to 100 MHz—telescopes typically take the form of directional antenna arrays resembling oversized television antennas, or large stationary reflectors equipped with movable focal points. Since the wavelengths involved are so long, the reflecting surfaces can be built from coarse wire mesh, something as simple as chicken wire, rather than solid metal. Move to shorter wavelengths and the picture changes dramatically: parabolic dish antennas become the dominant configuration. The angular resolution of any dish is governed by the ratio of its diameter to the wavelength under observation, a relationship that dictates just how large the structure must be to achieve useful detail. Instruments operating between 3 meters and 30 centimeters (100 MHz to 1 GHz) are generally well over 100 meters in diameter, while those probing wavelengths shorter than 30 centimeters (above 1 GHz) range from 3 to 90 meters across.

The Fight for Quiet Frequencies

Radio waves arriving from planets, stars, nebulae, and distant galaxies are extraordinarily faint, which is why radio telescopes demand both very large collecting antennas and extremely sensitive receiving electronics. That sensitivity creates a vulnerability: any nearby electromagnetic noise can drown out the signal. For this reason, radio observatories are deliberately sited far from major population centers, keeping them away from the constant electromagnetic interference generated by radio and television broadcasts, radar systems, motor vehicles, and countless other electronic devices. The problem is intensifying as more of the radio spectrum gets allocated to commercial and military communications. In the United States, a National Radio Quiet Zone exists to protect observation bands. Internationally, the Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science coordinates negotiations to defend the portions of the spectrum most valuable for studying the universe. Key protected bands include Channel 37 (608–614 MHz), the 21-centimeter Hydrogen line at 1,420.40575177 MHz, the so-called Waterhole spanning 1,420 to 1,666 MHz, and specific allocations at 1,406 MHz and 430 MHz.

Monumental Scales and Pioneering Measurements

The drive to capture ever-fainter signals has produced instruments of staggering physical scale. China's Five-hundred-meter Aperture Spherical Telescope, completed in 2016, stands as the world's largest filled-aperture radio telescope, its 500-meter dish presenting a collecting area comparable to thirty football fields. At the other end of the engineering spectrum, the Arecibo Observatory housed multiple receivers that together covered the entire 1 to 10 GHz range, while the Wilkinson Microwave Anisotropy Probe mapped the cosmic microwave background radiation across five distinct frequency bands centered on 23, 33, 41, 61, and 94 GHz. The Hydrogen line at 1,420 MHz proved pivotal when the Big Ear telescope used it to detect the famous Wow! signal. Unlike their optical counterparts, radio telescopes impose no night-time restriction; they can observe the sky in full daylight as readily as under a star-filled night. Individual dishes may also be linked electronically into arrays, multiplying their effective collecting power and angular resolution far beyond what any single antenna could achieve.

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