Astrophysics is, as many astrophysicists will tell you, the story of everything. The nature and evolution of stars, galaxies, galaxy clusters, dark matter and dark energy—and our attempts to understand these things—allow us to pose the ultimate questions and reach for the ultimate answers. But the practitioners of these arts, as the late astronomer Vera Rubin wrote in her autobiography’s preface, “too seldom stress the enormity of our ignorance.”
“No one promised that we would live in the era that would unravel the mysteries of the cosmos,” Rubin wrote. And yet a new observatory named for her, opening its eyes soon, will get us closer than ever before to unraveling some of them. This will be possible because the Vera C. Rubin Observatory will do something revolutionary, rare and relatively old-fashioned: it will just look out at the universe and see what there is to see.
Perched on a mountaintop in the Chilean Andes, the telescope is fully assembled and operating, although scientists are not able to use it just yet. A few weeks of testing remain to ensure that its camera—the largest in astronomical history, with a more than 1.5-meter lens—is working as it should. Engineers are monitoring how Earth’s gravity causes the telescope’s three huge glass mirrors to sag and how this slight slumping will affect the collection and measurement of individual photons, including those that have traveled for billions of light-years to reach us. They are also monitoring how the 350-metric-ton telescope will rapidly pan across seven full moons’ worth of sky, stabilize and go completely still, and take two 15-second exposures before doing it all over again all night long.
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In this fashion, the scope plans to canvas the entire sky visible from Earth’s Southern Hemisphere every three nights, remaking an all-sky map over and over again and noticing how it changes. And computer scientists are finalizing plans for how to sift through 20 terabytes of data every night, which is 350 times more than the data collected by the vaunted James Webb Space Telescope each day. Others are making sure interesting objects or sudden cosmic surprises aren’t missed among Rubin Observatory’s constant stream of images. Software will search for differences between each map and send out an alert about each one; there could be as many as 10 million alerts a night about potential new objects or changes in the maps.
From finding Earth-grazing asteroids and tiny failed stars called brown dwarfs to studying the strangely smooth rotation of entire galaxies sculpted by dark matter, the Rubin Observatory’s mission will encompass the entire spectrum of visible-light astronomy. The telescope will continue mapping the sky for 10 years. It may be better poised to answer astrophysicists’ deepest questions than any observatory built to date.
“The potential for discovery is immense,” said Christian Aganze, a galactic archaeologist at Stanford University, who will use the observatory’s data to study the history of the Milky Way.
Put more specifically, Rubin Observatory will collect more data in its first year than has been collected from all telescopes in the combined history of humanity. It will double the amount of information available to astronomy—and to anyone trying to understand our place in the universe.
Summit staff install Rubin Observatory’s commissioning camera (ComCam) on the telescope.
Rubin Observatory/NSF/AURA/H. Stockebrand
The Rubin Observatory’s Mission
The observatory’s goal was not always so broad. Originally named the Large Synoptic Survey Telescope (LSST), the Rubin Observatory was initially proposed as a dark-matter hunter. Vera Rubin found the first hard evidence for what we now call dark matter, a gargantuan amount of invisible material that shapes the universe and the way galaxies move through it. She and her colleague, the late astronomer Kent Ford, were studying the dynamics of galaxies when they made the discovery in the 1970s.
In a spiral galaxy like our Milky Way, the galactic core contains more stars and hence gravity than the outer arms do. This should mean that the objects closer to the core spin around faster than the objects on the outskirts. By observing how stars move around and how their light appears shifted as a result, Rubin and Ford found that the stars on the outskirts were moving just as fast as the ones closer in. They found the phenomenon held across the dozens of galaxies they studied. This pattern defied explanation, unless there was some extra unseen material out there in the far reaches, causing the galaxy to rotate faster on what only appear to be the outer edges.
Such dark materials had been proposed in the 1930s, but Rubin’s findings showed the power they exerted over regular, visible matter and provided the first evidence that they existed. “What you see in a spiral galaxy is not what you get,” Rubin once wrote.
To date, no one has directly seen dark matter or come to understand its physical nature, including the particles that comprise it in the same way we know the electrons, protons and neutrons that make up regular matter, including galaxies, giraffes and us. Early plans for the LSST sought to shed light on dark matter by mapping its distribution throughout the universe via its gravitational effects. Astronomers also wanted to study how the cosmos is expanding through the work of an equally mysterious companion force called dark energy. But as design on the telescope systems began, astronomers quickly realized the LSST could do much more than study dark matter—it could study almost anything, seen or unseen.
“It is not a telescope that you will be sending proposals saying, ‘I want to look over here.’ The purpose is the survey,” says Guillem Megias Homar, a doctoral student at Stanford University and member of the telescope team.
The Rubin Observatory’s camera is housed in a cryostat assembly.
Andy Freeberg/SLAC National Accelerator Laboratory
Mirrors and Cameras
The open-ended surveying mission is a boon for astronomers, but it comes with intense design challenges. The telescope has to move across a swath of sky in just a few seconds and stop jittering almost immediately so that its images are clear. At other observatories, where astronomers choose targets ahead of time and plan what they’re looking for, telescope engineers have maybe 10 minutes to stop the glass from wobbling in between taking images. Rubin Observatory gets five seconds, says Sandrine Thomas of the U.S. National Science Foundation National Optical-Infrared Astronomy Research Laboratory (NOIRLab), a deputy director of the observatory’s construction.
“When you want to move that amount of mass very quickly and be stable, you can’t have a very long telescope; otherwise the top wobbles,” she says. “The light cannot go a long way before it loses focus, and that creates a lot of challenges.”
To make the system more compact, Rubin Observatory’s main telescope has a unique three-mirror structure. The primary and tertiary mirrors were fabricated to share the same piece of glass. Light bounces off the ring-shaped primary mirror and shines upward into a separate, secondary mirror, itself the largest convex mirror ever made. The secondary mirror again bounces the light back toward the tertiary mirror, which is inside the primary mirror’s outer ring. The third mirror reflects light into the camera’s sensitive detectors. The primary mirror and tertiary mirror combined give the telescope a collecting area of 6.67 meters. The secondary mirror has a 1.8-meter hole in the middle that the camera and its electronics fit into. And the tertiary mirror has a hole, too, for equipment designed to align the telescope and stop it from wobbling. The camera is a 10-meter-by-10-meter steel cube, small and compact.
Margaux Lopez, a mechanical engineer, started working for the SLAC National Accelerator Laboratory after graduating from the California Institute of Technology in 2015 and has been working on the camera ever since. “The point of this project is to collect a wild amount of data,” she says. “How we actually do that is to see more of the sky at once, take more images at night and get more detail in each photo—that’s the trifecta.”
Astronomers often use the full moon’s disk to describe a telescope’s field of view; for an optical telescope, Rubin Observatory’s view is unparalleled. The Hubble Space Telescope observes about one percent of a full moon, and JWST observes about 75 percent of the moon’s disk. Each Rubin Observatory image captures an area about 45 times the size of the full moon, Lopez says.
“We are just seeing a wildly larger amount of sky with every image we take and getting an equal or greater amount of detail, even though the field of view is so big,” she says.
The camera can take images in six filters, from the near ultraviolet to the near-infrared range. But astronomers must understand how the camera itself affects the images. Dark matter distorts the direction of photons streaming from distant galaxies, but so does the optics system, Megias Homar says.
“We really need to be sure about this. How is it affecting the light itself? If there is turbulence in the atmosphere or in the optics, a dot can become blurry,” Megias Homar says. He spent his doctoral program working on Rubin Observatory’s optics system to understand this issue better.
A commissioning camera, ComCam—a smaller, simpler version of the full LSST Camera—was used for testing Rubin Observatory before its full camera was installed.
Rubin Observatory/NSF/AURA/H. Stockebrand
Mountaintop Observing
After construction was complete, the telescope parts had to travel from California and Arizona to the top of Cerro Pachón, an 8,799-foot, seismically active peak in the Chilean Andes. Lopez and her colleagues chartered a Boeing 747 freighter jet to bring the camera from San Francisco to Santiago, Chile, in May 2024. The subsequent trip to La Serena, the city nearest the telescope’s mountaintop home, required a 12-hour truck ride. Lopez monitored every step of the journey, even dealing with a trucking strike that threatened to blockade the route to Cerro Pachón. Finally, the camera made it to the literal mountaintop, where Lopez took it apart and checked everything. Teams of engineers, including Megias Homar, spent months testing the camera and its companion commissioning camera, a smaller version of the real thing that astronomers used to test all telescope systems, which went live on the sky in October 2024. The engineers shifted to nighttime work, sleeping during sunlight hours like astronomers do when they are at the observatory.
“That was the first time we saw images. For a whole month, I was going to sleep at 6 A.M. and feeling like an astronomer,” Megias Homar says. He worked with engineers and astronomers who have been planning and designing the LSST project since its inception. One person told Megias Homar they began working on it in 1996.
“I was born in 1997, so that was really humbling,” he says.
Thomas has been part of the team for 10 years but got her start as an observer on a mountain next door to Rubin Observatory. “When I joined the project, I did not appreciate how different this discovery machine or even this observatory was. I am coming from a normal, classical type of observing, which is submitting your proposal, maybe getting some time, maybe not,” she recalls. “Bringing this amount of data to the community, to me, is just extremely rich.”
For astronomers and astrophysicists, the richness is almost giddying. Rubin Observatory’s 10-year main mission will provide a sort of time-lapse movie of the cosmos that will show other observatories where to look for new discoveries. A decade is not a long time in the history of the universe, but it is longer than anyone has ever stared at the sky.
Telescope’s First Light
Galactic archaeologists like Aganze are hoping to study the history of our galaxy and how dark matter might be sculpting its evolution, just like the distant spiral galaxies Vera Rubin glimpsed a half century ago. Recent surveys from telescopes like the Gaia satellite show that the Milky Way is surrounded by streams of stars that might shed light on the dark matter halo that surrounds us. Galaxy streams can help astronomers understand when galaxy formation shuts off or how much dark matter must be around a smattering of stars for it to agglomerate into a galaxy. With Rubin Observatory, researchers should be able to see all the stars in a galactic stream, detect the stream’s shape and even figure out what its associated dark matter must be like, Aganze says. And we could potentially do this for 100 or 200 galaxy streams around the Milky Way.
“If little dark matter clumps mess up the stars, we should be able to see it. We should be even able to put constraints on the dark matter—is it cold, warm or self-interacting?” Aganze says, describing three main theories for dark matter’s properties. “[Rubin Observatory] is going to be great for this kind of science. We should definitely be able to march forward the limits of galaxy formation and the little dark matter halos.”
The observatory will also find millions of new objects in our solar system, including 90 percent of all large asteroids that fly past Earth and thousands of tiny worlds far beyond Neptune’s orbit. By essentially producing a time-lapse video, the observatory will unveil countless new transient and time-sensitive phenomena in the distant cosmos, such as quasars streaming from supermassive black holes. It will carefully scrutinize a special type of exploding binary star called a type Ia supernova that is essential for astronomy measurements and can shed more light on the nature of dark energy.
Astronomers plan to share images from the camera—“first look,” as they are calling it—on June 23. Megias Homar says he is excited for the weeks ahead but admits that his first concern will be the optical system.
“I will be worried that this thing is working; that is where my mind is going to go first,” he says. And then he will turn his attention to the main mission: looking out at the cosmos.
Astronomers eager to use the Rubin Observatory frequently talk about the value of just looking at the universe. Basic research is a public good, they say, that can provide new insight into our history while improving our shared future.
“It feels very much like a project based on curiosity,” Lopez says. “Humans have always wanted to go to the top of the tallest mountain or the furthest reaches of the ocean, and this feels like one of those types of things. Let’s create the coolest instrument we can to find out more about who we are.”
Nobody ever promised that this generation of astronomers could unravel the mysteries of the cosmos, as Rubin herself reminds us. But right now we live in a time when we can try.
