After the Second World War, 1940s America was awash with radar equipment and experienced technicians who could make it work. That set the scene for the birth of a new field of science – radio astronomy.
In 1944, the Dutch astronomer Hendrick van de Hulst predicted that interstellar hydrogen ought to emit electromagnetic radiation with a specific wavelength. He noted that a hydrogen atom contains only a proton and an electron that both have a property known as spin. These particles can spin in the same direction or in opposite directions and van de Hulst realized there must be a small energy difference between these two states.
So any hydrogen atom flipping from the more energetic state to the lower energy state must emit a photon, which he calculated would have a wavelength of 21 centimeters and a frequency of 1420 Megahertz. Thatâ€s roughly the same microwave wavelength that radar operated in.
In 1951, astronomers working at Harvard University in Cambridge detected this radiation for the first time. And since the universe is filled with hydrogen in varying densities, the ability to map out this distribution kickstarted the nascent field of radio astronomy.
Galactic Rotation
Since then, astronomers have used it to determine the structure of the Milky Way, to measure the rotation of other galaxies and to probe the role of hydrogen in the early universe shortly after the Big Bang.
But unlike ordinary astronomy in which amateurs have played an important role, radio astronomy has been inaccessible to all but the best funded institutions. Now that looks set to change thanks to the work of Jack Phelps, who has published the design of a radio telescope that anybody can build in their backyard for a few hundred dollars.
In principle, Phelps†device is straightforward. It consists of a 1-meter parabolic dish of the type used for satellite TV reception. This focuses analogue radio signals from the sky and sends them through a low-noise amplifier that boosts the signal, a bandpass filter that rejects signals outside the frequencies of interest and then through another low-noise amplifier.
At this point, the signal is digitized by a software-defined radio running on a Raspberry Pi 4 microcomputer with 8GB of RAM and a 64-bit quadcore processor running at 1.5 GHz, all powered by a power-over-ethernet cable to keep noise to a minimum.
The Pi runs a bespoke operating system developed by Glen Langston at the US National Science Foundation, specifically for observing the 21-centimeter hydrogen line and processing the data from it.
Phelps mounted all this equipment on top of his house, which raised another problem. Ordinary suburban houses are filled with electronic devices and equipment that can sometimes emit electromagnetic noise at exactly the frequencies that radio telescopes aim to detect.
To mitigate this problem, he stored all the signal processing equipment in boxes covered with kitchen foil, grounded to Earth, that would prevent the penetration of electromagnetic interference. “The foil not only protected against EMI, but also provided thermal insulation,†says Phelps.
Secondhand Bargains
The total cost of this set-up is less than $200-$400, with secondhand equipment or repurposed devices significantly reducing the price.
Phelps calculated the area of the sky dish can observe. “The calculated beamwidth of the antenna is about 14.78 degrees,†he says. So by scanning the dish to take observations at many different points, he is able to build up a picture of parts of the sky.
And the results are impressive. Phelps pointed the dish at the Galactic Centre within the Sagittarius arm, which is known to have star-forming clouds rich in hydrogen. The instrument picked up hydrogen peaks in these areas and Phelps was even able to observe a small redshift suggesting that these clouds must be moving away from Earth.
These results are exactly as expected. “The consistency of these observations with the expected structure of the Sagittarius Arm supports the accuracy of the data, despite the potential atmospheric challenges,†says Phelps.
He finds one or two issues with atmospheric noise, but it seems clear that his backyard radio telescope is a seriously useful piece of kit. “Higher elevations provide clearer signals, while lower elevations show more atmospheric noise, but the overall structure of the Milky Wayâ€s spiral arms is still detectable at all angles,†concludes Phelps.
Thatâ€s interesting work that will hopefully spawn plenty of followers. Whatâ€s needed next is a community of amateur radio astronomers who can begin gathering data that will complement the work of their professional colleagues, just as amateur astronomers working in the visible part of the spectrum have done for hundreds of years.
So if you have room in your backyard and a spare satellite dish lying around, why not give it a try? Van de Hulst would surely be impressed.
Ref: Galactic Neutral Hydrogen Structures Spectroscopy and Kinematics: Designing a Home Radio Telescope for 21 cm Emission :arxiv.org/abs/2411.00057
