Assistant Professor Meredith MacGregor and NIST Physicist Jake Connors taught their graduate students how to build and use radio horn antennas to locate neutral hydrogen in space

Meredith MacGregor, CU Boulder assistant professor of astrophysics, and Jake Connors, a physicist at the National Institute of Standards and Technology (NIST), wanted to teach their graduate students in Astrophysics 6000 (ASTR 6000) the basics of radio astronomy. But how?

“Jake and I had been trying to brainstorm how we would teach radio astronomy in a way that would get students excited and actually learn it,” says MacGregor.  

Then they had an idea—a brainwave, as it were: Why not teach their students how to build and then use pyramidal horn antennas as do-it-yourself radio telescopes? 

“It just seemed better to do something creative instead of sitting in a classroom and lecturing people,” says MacGregor.

Connors agrees. A hands-on approach, he says, “is a way to reach those students who aren’t keen on learning from people writing on the board.”  

So, they and their students got to work.

The method

State-of-the-art radio antennas, like the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, can cost billions of dollars to construct, but perfectly serviceable ones can be built for less than $100. All it takes is a bit of ingenuity. 

For the horn, which acts as a funnel for the radio signal, MacGregor and Connors’s students used metallized home insulation. “We literally drove to Home Depot, bought pieces of insulation, strapped them to the roof of a car and drove them to campus,” recalls MacGregor. 

And for the waveguide, which picks up the signal at the narrow end of the horn, they used cans of olive oil from Whole Foods. “The olive oil cans turned out to be exactly the right size, conveniently enough,” says MacGregor. “Whole Foods makes really great olive oil cans.” 

(They emptied the oil from the cans but wasted none of it. “My pantry is full of olive oil right now,” says MacGregor.)

They also purchased a software-defined radio, or SDR, from Amazon, which Connors says functioned a bit like a microphone, amplifying, sampling and digitizing the astronomical signals before sending them to a computer.   

All in all, the students of ASTR 6000 built three antennas. And when they pointed them skyward, they searched for one thing in particular: neutral hydrogen in the arms of the Milky Way. 

The element

Hydrogen has one proton and one electron, each of which has a property called spin, MacGregor explains. These spins can be in alignment, with both going up or down, or they can be opposed. 

But when the spin of the electron flips, it gives off a 21-centimeter radio wavelength, otherwise known as the hydrogen line, the finding of which provides a kind of window view upon the galaxy. 

“Our galaxy has this spiral-arm structure,” says MacGregor, “and in those arms are a bunch of gas clouds that are mostly made up of hydrogen.” 

Connors adds that these gas clouds are moving relative to the earth because of the way the galaxy rotates.

“They’re orbiting the same supermassive black hole at the center of the galaxy that we are. But because they’re moving at a different distance from the center of the galaxy than we are, they’re moving at a different velocity potentially.” 

These different velocities then create different Doppler shifts, which the horn antennas detect. 

“Much like if you stand on a sidewalk and hear an ambulance go by, when it’s coming toward you, the ambulance siren sounds really high pitched, and then, as it goes past you, the pitch drops,” says Connors. 

“It’s that same Doppler shift, except for light. We’re actually observing the light emitted from those hydrogen clouds.” 

Different gas clouds produce different Doppler shifts, and those different Doppler shifts represent different velocities. 

“And then you can say, ‘I looked in this line of sight, and I saw these different arms of the galaxy.’ And then you look at another line of sight, and you’ll see some different distribution of velocities. And so you can basically map the entire galaxy,” says MacGregor.

The bigger picture

Student Jay Chittidi says building the radio antennas “was one of the most engaging hands-on labs that I have had as a grad student. This approach really solidified our understanding of the basics of radio astronomy, because we had to plan with them in mind when building and observing with our antennas.”

But MacGregor and Connors say they believe the lessons gleaned from this exercise go beyond hydrogen gas clouds and radio antennas specifically.  

“I’m hoping to ignite a passion in the students for doing instrumentation,” says Connors. “There is a dearth of people who are out there building the instruments to enable observations to take place. Seeing both sides of how astrophysics works is really valuable for students’ careers and understanding.” 

“We’d really like students to have an understanding of how we do observational science on all levels,” says MacGregor.

She points out that the initial measurement of the 21-centimeter line and subsequent mapping of the galaxy were major breakthroughs in physics, and yet the scientists responsible for those breakthroughs, Harold Ewen and Edward Purcell, used a horn antenna very much like the ones built in ASTR 6000. 

“And I think that’s a very powerful lesson.”