Caltech Readies to Build World's Most Sensitive Radio Telescope

Our cosmos is awash with radio waves, originating from fierce jets blasting out of distant black holes, blinking dead stars closer to home, and many other exotic objects. To observe radio waves—which possess wavelengths hundreds of thousands of times longer than visible light—astronomers use two types of telescopes: huge single dishes, such as the giant FAST (Five-hundred-meter Aperture Spherical Telescope) in China, and arrays of many dishes, such as the National Radio Astronomy Observatory Very Large Array in New Mexico, whose sprawling 27 dishes were famously featured in the movie Contact.

While the giant single dishes are very sensitive, which means they excel at detecting faint radio waves from across the cosmos, arrays of many dishes can take the sharpest images.

Now, researchers at Caltech are getting ready to build a radio telescope that has both exquisite sensitivity and the ability to take crisp pictures. The Caltech-led Deep Synoptic Array (DSA) recently completed its final design review with Schmidt Sciences, which announced in January that it is funding the project.

The milestone paves the way for construction to begin. Planned for a remote valley in Nevada, the DSA will consist of 1,650 radio dishes, each slightly more than 6 meters in diameter—by far the most dishes to make up a radio array. The array will span an area of about 20 x 16 kilometers. The DSA team plans to build the telescope by 2029, with science operations commencing soon after.

Once completed, the DSA will tout an impressive list of superlatives: It will be the most sensitive radio telescope ever built, produce the highest-quality radio images, and survey the sky 100 times faster than any other radio telescope worldwide.

"The DSA will survey the entire visible sky several times in its first five years at unprecedented speeds," says Gregg Hallinan, principal investigator of DSA, professor of astronomy at Caltech, and director of Caltech's Owens Valley Radio Observatory (OVRO). "While all other radio telescopes combined have so far found about 20 million radio sources, the DSA will match that in the first day of operations. By the end of its initial survey, it will have discovered about 1 billion new radio sources."

The DSA will transform radio astronomy, a discipline Caltech helped pioneer in the US in the 1950s to explore the radio portion of the electromagnetic spectrum. The telescope will discover radio emission from millions of stars, galaxies, and other cosmic characters that shine, pulse, and explode with radio light. It will address the mysteries of black holes, pulsars (magnetized spinning dead stars), and fast radio bursts, or FRBs (brief and powerful flashes of radio waves often originating from very distant sources). It will also probe the physics of dark matter and gravity, and it will measure the structure and expansion of our universe.

"Radio astronomy is about to go from sketch to photograph," says Vikram Ravi, the co-principal investigator of the DSA and a professor of astronomy at Caltech. "The DSA is looking at a far larger volume of the universe far more often than any other telescope. I'm excited for all the discoveries we know we will make, and the ones we don't expect."

First-Ever Radio Camera

In addition to its impressive sensitivity and survey speed, the DSA will be uniquely capable of making images in real-time. These images will be immediately accessible to the worldwide astronomical community. Whereas other radio telescopes typically take up to months for their data to be processed and turned into images, the DSA's numerous radio dishes will feed into a supercomputer that creates images instantly. In essence, the DSA will host the world's first "radio camera."

This radio camera technology is in fact the key reason that the DSA is even possible. As the number of dishes in a radio array goes up, so too does the amount of data collected. Having 1,650 dishes creates a torrent of raw data at a rate equivalent to all current US internet traffic, an amount too big to store practically. Instead, by creating images in real time, the radio camera tames the flood of data, making it more manageable.

"Without the radio camera, we would have to store 100 exabytes of data [100 billion gigabytes] to complete our survey," Hallinan explains. "This would require 5 million hard drives in a multi-billion-dollar facility the size of multiple football fields. The radio camera solves this problem."

What is more, the reason that the DSA can process the data in real time is precisely because it has so many dishes.

"Having thousands of dishes is both the cause and solution of the problem," Hallinan says. "With 1,650 dishes, we hit this long-sought threshold where we have enough dishes to essentially measure all the information about the sky, and this allows us to process the data more easily at the telescope site."

The DSA's radio camera will convert the raw data to images in real time with the help of an off-site supercomputer built from cutting-edge rack-scale Graphics Processing Units (GPUs) built by Nvidia. In the end, the radio camera will mean that only tens of petabytes (a petabyte equals 1 million gigabytes) are archived per year instead of the 100 exabytes collected.

The radio camera images will be given freely to the public with no proprietary period.

"We want the whole world to also have access to the data just as quickly as we do," says Caltech's Katie Jameson, the DSA lead project manager. "The DSA functions like a photo lab that is developing these radio images in real time for all to use. Everyone will be able to probe the wonders of the radio universe right after the data are collected."

Along with a real-time radio camera, the DSA will have the ability to autonomously search the sky at 1,000 frames per second using a parallel system called the Chronoscope. "While the radio camera makes exquisite images of the sky, the Chronoscope is like making movies with your phone to search the sky for pulsars, FRBs, and unanticipated discoveries," Ravi says.

A Bonanza of Radio Dishes

Building an array of 1,650 dishes could have turned out to be "prohibitively expensive," as Hallinan says, if it were not for several innovations. Some of these advances involved improving the performance of the dishes themselves, an effort that has been in development over the past decade using the dishes of Caltech's DSA-110 at OVRO.

"The goal was to get the cost down while creating high-performing dishes with surface and pointing accuracies that enable the radio camera, and to do so reliably and efficiently at scale," says Caltech's Francois Kapp, the DSA lead project engineer. "DSA would not be feasible without many innovations and optimizations in the design of the dishes by the team at OVRO." DSA has partnered with Mtex Antenna Technology GmbH in Germany in the final design and manufacturing of the 1,650 dishes.

Another striking innovation involves the dishes' receivers—small devices that sit at the focus of the dishes and amplify feeble radio signals from the cosmos. Typically, receivers need to be chilled to reduce their own internal noise, a hiss of radio waves that interferes with observations. But chilling 1,650 receivers would have required each dish to be outfitted with expensive cryogenic coolers. Instead, Caltech radio astronomer Sandy Weinreb figured out a new way to design the amplifiers such that cooling them will not be required.

"Now, a team led by Steve Padin [research professor of physics at Caltech] has completed the final design of these revolutionary electronics," Hallinan says.

The room-temperature receivers were also tested at DSA-110, while the radio camera technology was developed with the help of OVRO's Long Wavelength Array, which studies space and solar weather, exoplanets, and more. These two pathfinder arrays were funded by the National Science Foundation (NSF).

Another cost-saving design element came from a surprising source: cake pans. It turns out that metal cake pans are the perfect shape to serve as components of the feed, which converts electromagnetic waves to electrical signals. The team has hired a baking pan company called Fat Daddio's to manufacture thousands of cake pans for the DSA. The company even built a special tube structure to cut out holes in the bottom of the pans, transforming them into rings. "It's all about metal fabrication, and this is something Fat Daddio's has a lot of experience in!" Kapp says.

Optical fiber is also key to making the DSA happen. The many dishes of the DSA and the off-site supercomputer will be connected via a complex network of optical fiber cables designed and to be installed by Praxis Broadband, a company that has been part of the DSA team since the early days of the project.

The German Center for Astrophysics (DZA) is preparing to join the DSA project later this year and is about to build an eight-element test array for the DSA close to its headquarters in eastern Germany. Using identical technical components, it will be a technology demonstrator and software platform for the DSA project.

Fireworks in the Radio Sky

This decade has seen the rise of telescope surveys that image our ever-changing sky, cataloging what Hallinan calls "things that go bump in the night," including millions of zippy asteroids, exploding stars, and burbling black holes. For instance, the Zwicky Transient Facility (ZTF), which operates from Caltech's Palomar Observatory in San Diego County, has detected millions of changing space objects since it began operations in 2017, and the more recent Vera C. Rubin Observatory in Chile, funded by the NSF and the U.S. Department of Energy, is now up and surveying the skies at even faster speeds and greater depths.

The DSA will serve as the radio counterpart to the optical surveys conducted by Rubin and ZTF, as well as the Argus Array, another upcoming project within the Eric and Wendy Schmidt Observatory System. Of the billion sources of radio light the project will detect over its five-year survey, millions of these will include new discoveries of FRBs, supernovae, star-consuming black holes, and other exotic events. The DSA will have the ability to detect more than 100,000 intensely powerful flashes of radio light from FRBs and to localize them to their home galaxies.

"In addition to giving us a full picture of the true nature of FRBs, this will transform cosmology," Ravi says. "We will significantly improve measurements of evolving dark energy and the mass of the neutrino, both huge mysteries in basic physics."

DSA will also provide unprecedented maps of gas and star formation in galaxies. "This will enable astronomers, but also the interested public, to obtain very deep radio images of their favorite galaxy, complementing those already available at other wavelengths," says Fabian Walter, the DSA project scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany.

The Race to Catch Neutron Star Mergers

Like other survey telescopes, the DSA has a very large field of view and that means it will be able to rapidly search large volumes of sky. This trait makes it a powerful resource for following up on dramatic cosmic explosions, such as mergers between two neutron stars, which fling heavy metals such as gold and platinum into space.

So far, only one neutron star merger has been seen to date in both gravitational waves (ripples in space and time) and light waves, a historic event known as GW170817. In 2017, the NSF-funded LIGO (Laser Interferometer Gravitational-wave Observatory) and its sister observatory in Europe, Virgo, spotted the neutron star merger first in gravitational waves, prompting light-based telescopes to follow up and quickly pin down the location of the event. With the whereabouts known, other telescopes could then turn their gaze and witness the rare spectacle unfold. Radio observations were decisive in establishing that GW170817 launched a relativistic, or near-light-speed, jet.

In the future, as LIGO and its partners pick up gravitational waves from additional neutron star collisions, the DSA will be particularly helpful in the race to pinpoint the location of these events. The radio array will be crucial for localizing faint gravitational-wave signals from deep space that optical telescopes might not be able to see. Its observations will provide a better understanding of how neutron star mergers evolve and may even reveal how they launch relativistic jets created in the blasts.

Radio Beacons

Other exotic characters to be revealed by the DSA will include more than 20,000 new pulsars. These spinning neutron stars send out beams of radio waves that rotate like lighthouse beacons. Because they are so regular in their timing, pulsars can be used to look for gravitational waves. As the waves wash through space, they stretch and squeeze the fabric of space, altering the amount of time a pulsar's signal would take to reach Earth.

While LIGO detects gravitational waves from mergers of stellar-mass black holes and neutron stars, pulsar timing can catch a background sea of gravitational waves emanating from pairs of much larger, supermassive black holes that are spiraling together. The DSA will work with other telescopes to look for this slow-rolling gravitational sea pervading our cosmos. It is also uniquely capable of making the first detection of gravitational waves from a single, particularly loud pair of spiraling supermassive black holes.

"The science that can be done is endless," Hallinan says. "There will be enough discoveries to occupy every radio astronomer on the planet. With fully public, science-ready data, some of these discoveries may even be made by a high-school student with a clever idea. The radio sky is the limit!"

The DSA is led by Caltech and funded by Schmidt Sciences. It is part of the Eric and Wendy Schmidt Observatory System, which includes three other telescope projects outside of Caltech—the Argus Array, the Large Fiber Array Spectroscopic Telescope (LFAST), and the Lazuli Space Observatory. Two pathfinder projects that led to the DSA, the DSA-110 and the OVRO Long Wavelength Array, were funded by the NSF.