When the immense Arecibo radio telescope in Puerto Rico collapsed in 2020, it left gaping holes in astronomy. Now, a team from the California Institute of Technology (Caltech) hopes to address some of the gaps with a very different instrument: a tightly packed array of relatively inexpensive radio dishes that aims to quickly image radio sources across wide swaths of the sky. A nearly completed prototype array in California that the team calls a “radio camera” is already locating dozens of the distant, enigmatic eruptions called fast radio bursts (FRBs). Next year, the team hopes to begin construction on a much larger array with 2000 dishes that, together, will match the size of Arecibo.

Maura McLaughlin of West Virginia University is a leader of NANOGrav (the North American Nanohertz Observatory for Gravitational Waves), an effort to search for gravitational waves from supermassive black holes that relied on Arecibo for half its data. She says they took “a big sensitivity hit” when it was lost. “We really need a new telescope with a similar collecting area,” she says, and Caltech’s planned Deep Synoptic Array (DSA) fits that bill. “It will be a game changer.”

To gain sensitivity, radio astronomers can build big dishes like Arecibo or arrays of smaller dishes. But in most such arrays, the dishes are widely spaced, which sharpens their resolution but creates “a data deluge problem,” says Caltech’s Gregg Hallinan, DSA principal investigator (PI). Producing an image from a scattered array is like looking through a fragmented mirror, he says, and recreating the information from the missing parts is a complex nonlinear process known as deconvolution that can take weeks—or even years.

Many astronomers just want to regularly survey the sky for new objects or monitor sources for subtle changes without a heavy processing burden. Caltech’s solution, Hallinan says, is to “fill the mirror up” by packing low-cost dishes together. That makes deconvolution easier and should enable DSA to construct images in real time.

The team has nearly finished assembling its prototype, the DSA-110, a T-shaped array of 95 dishes spaced 1 meter apart at Caltech’s Owens Valley Radio Observatory in California plus another 15 “outriggers” spread out more than 1 kilometer away. To keep construction costs to $4 million, the instrument uses commercially available 4.6-meter dishes, homemade amplifiers, and wave-channeling feeds fashioned out of cake tins. Most radio telescopes require expensive cryogenic cooling to reduce amplifier noise, but Caltech’s engineers have squeezed similar performance out of room-temperature circuits. (Co-PI Vikram Ravi admits they perform less well in the summer heat.)

With a wide field of view, DSA-110 is good at detecting FRBs, intense blasts of radio waves lasting only milliseconds, coming from all over the sky. Several thousand have been detected, but little more than a dozen have been traced to their home galaxies, which might hold clues to what is powering the bursts. DSA-110 aims to localize many more. If a burst is detected, data from the outrigger dishes allow the telescope to zoom in and pin the FRB to its galaxy.

During 2022, with more than half the dishes in place including the outriggers, DSA-110 identified source galaxies for about 20 FRBs, overtaking the number of localized sources found by all other telescopes. When completed this summer, it should localize a couple of FRBs every week, Ravi says. “It’s going to be a lot of fun.”

The results so far are already confounding theorists. They predicted FRBs are most likely to come from galaxies that are rapidly forging new stars. Such galaxies abound in massive stars that quickly run out of fusion fuel and collapse into tiny stellar remnants rippling with energy called magnetars—the favored engines for blasting out FRBs. But DSA-110 has found FRBs in quiescent galaxies whose magnetars would have long fizzled out. That suggests FRBs might have other sources besides magnetars, says Victoria Kaspi of McGill University. Just  this week in Nature Astronomy, researchers reported an FRB that appeared to come from the merger of two neutron stars. “Having many FRBs localized is important,” Kaspi says. “We may be able to see different populations [of FRBs] from different sources."

Meanwhile, the Caltech team is gearing up for the next phase: DSA-2000. At 19 by 15 kilometers, it will be too big for Owens Valley so the team is looking at Hot Creek Valley in Nevada, a sparsely populated, radio quiet region. To keep costs low, the Caltech team plans to make its own 5-meter dishes by molding sheets of aluminum. Although DSA-2000 won’t be as sensitive as other planned radio observatories, such as the Square Kilometre Array in South Africa and Australia, it will beat them on survey speed. Existing surveys have logged 10 million radio sources across the sky, Hallinan says. DSA-2000 will boost that number 100-fold to 1 billion, giving astronomers, for example, a better picture of how galaxies form and grow, and allowing them to capture fleeting sources like merging neutron stars over a much wider volume.

Later this year, the team will apply to the U.S. National Science Foundation to supplement the private sources that have funded development work. If the team can raise the $144 million needed to build the array, DSA-2000 could start logging tens of thousands of FRBs in 2026. That could enable astronomers to start using FRBs as a mapping tool. As the compact pulses move through space, they get smeared by the gas they pass through—giving astronomers a clue to the location of gas around and between galaxies that is normally invisible to telescopes. Astronomers don’t know where half the normal matter in the universe is; FRBs could help them find it.

One-quarter of its time will be devoted to eavesdropping on another hidden component of the universe: the colossal black holes lurking at the centers of galaxies with masses of millions or billions of suns. When galaxies merge, the newly formed galaxy ought to end up with two of these lumbering giants, circling each other warily and churning out long, languorous gravitational waves. Detectors on Earth have detected the shorter, sharper waves generated by collisions of star-size black holes. But it takes a detector light-years across to pick up these long waves. NANOGrav’s strategy is to studiously observe pulsars, spinning stellar fossils that emit metronomic radio pulses hundreds of times a second. A passing gravitational wave would slightly shift a pulsar’s repetition rate as it ripples space between the pulsar and Earth.

More than a decade of watching several dozen pulsars has yet to turn up a firm detection. But with DSA-2000 monitoring more pulsars, more accurately and more often, a signal might emerge, says NANOGrav member Chiara Mingarelli of the University of Connecticut, Storrs. That would open “a new frontier,” she says, revealing giant black holes performing a hidden pas de deux. “DSA-2000 will be transformational for gravitational wave astronomy.”