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For the next decade, the 350-tonne Simonyi Survey Telescope inside the Vera C. Rubin Observatory will change astronomy through the widest, fastest, and deepest survey of the sky ever performed.
Night is falling on Cerro Pachón.
Stray clouds reflect the last few rays of golden light as the sun dips below the horizon. I focus my camera across the summit to the westernmost peak of the mountain. Silhouetted within a dying blaze of red and orange light looms the sphinxlike shape of the Vera C. Rubin Observatory.
“Not bad,” says William O’Mullane, the observatory’s deputy project manager, amateur photographer, and master of understatement. We watch as the sky fades through reds and purples to a deep, velvety black. It’s my first night in Chile. For O’Mullane, and hundreds of other astronomers and engineers, it’s the culmination of years of work, as the Rubin Observatory is finally ready to go “on sky.”
Rubin is unlike any telescope ever built. Its exceptionally wide field of view, extreme speed, and massive digital camera will soon begin the 10-year Legacy Survey of Space and Time (LSST) across the entire southern sky. The result will be a high-resolution movie of how our solar system, galaxy, and universe change over time, along with hundreds of petabytes of data representing billions of celestial objects that have never been seen before.
Stars begin to appear overhead, and O’Mullane and I pack up our cameras. It’s astronomical twilight, and after nearly 30 years, it’s time for Rubin to get to work.
On 23 June, the Vera C. Rubin Observatory released the first batch of images to the public. One of them, shown here, features a small section of the Virgo cluster of galaxies. Visible are two prominent spiral galaxies (lower right), three merging galaxies (upper right), several groups of distant galaxies, and many stars in the Milky Way galaxy. Created from over 10 hours of observing data, this image represents less than 2 percent of the field of view of a single Rubin image.
NSF-DOE Rubin Observatory
A second image reveals clouds of gas and dust in the Trifid and Lagoon nebulae, located several thousand light-years from Earth. It combines 678 images taken by the Rubin Observatory over just seven hours, revealing faint details—like nebular gas and dust—that would otherwise be invisible.
NSF-DOE Rubin Observatory
Engineering the Simonyi Survey Telescope
The top of Cerro Pachón is not a big place. Spanning about 1.5 kilometers at 2,647 meters of elevation, its three peaks are home to the Southern Astrophysical Research Telescope (SOAR), the Gemini South Telescope, and for the last decade, the Vera Rubin Observatory construction site. An hour’s flight north of the Chilean capital of Santiago, these foothills of the Andes offer uniquely stable weather. The Humboldt Current flows just offshore, cooling the surface temperature of the Pacific Ocean enough to minimize atmospheric moisture, resulting in some of the best “seeing,” as astronomers put it, in the world.
GyGinfographics
It’s a complicated but exciting time to be visiting. It’s mid-April of 2025, and I’ve arrived just a few days before “first photon,” when light from the night sky will travel through the completed telescope and into its camera for the first time. In the control room on the second floor, engineers and astronomers make plans for the evening’s tests. O’Mullane and I head up into a high bay that contains the silvering chamber for the telescope’s mirrors and a clean room for the camera and its filters. Increasingly exhausting flights of stairs lead to the massive pier on which the telescope sits, and then up again into the dome.
I suddenly feel very, very small. The Simonyi Survey Telescope towers above us—350 tonnes of steel and glass, nestled within the 30-meter-wide, 650-tonne dome. One final flight of stairs and we’re standing on the telescope platform. In its parked position, the telescope is pointed at horizon, meaning that it’s looking straight at me as I step in front of it and peer inside.
The light of the full moon highlights the Rubin observatory building, the orientation and tiered layers of which were developed through computational fluid dynamics to stabilize airflow around the telescope.
Enrico Sacchetti
The telescope’s enormous 8.4-meter primary mirror is so flawlessly reflective that it’s essentially invisible. Made of a single piece of low-expansion borosilicate glass covered in a 120-nanometer-thick layer of pure silver, the huge mirror acts as two different mirrors, with a more pronounced curvature toward the center. Standing this close means that different reflections of the mirrors, the camera, and the structure of the telescope all clash with one another in a way that shifts every time I move. I feel like if I can somehow look at it in just the right way, it will all make sense. But I can’t, and it doesn’t.
The three-mirror anastigmat design of the telescope maximizes image quality and field of view while remaining compact and nimble.
GyGinfographics
I’m rescued from madness by O’Mullane snapping photos next to me. “Why?” I ask him. “You see this every day, right?”
“This has never been seen before,” he tells me. “It’s the first time, ever, that the lens cover has been off the camera since it’s been on the telescope.” Indeed, deep inside the nested reflections I can see a blue circle, the r-band filter within the camera itself. As of today, it’s ready to capture the universe.
The blue r-band filter within the camera is reflected in the M3 mirror in this photo of the telescope parked at horizon.
Enrico Sacchetti
Hernán Stockebrand/NSF-DOE Rubin Observatory
Rubin’s Wide View Unveils the Universe
Back down in the control room, I find director of construction Željko Ivezić. He’s just come up from the summit hotel, which has several dozen rooms for lucky visitors like myself, plus a few even luckier staff members. The rest of the staff commutes daily from the coastal town of La Serena, a 4-hour round trip.
To me, the summit hotel seems luxurious for lodgings at the top of a remote mountain. But Ivezić has a slightly different perspective. “The European-funded telescopes,” he grumbles, “have swimming pools at their hotels. And they serve wine with lunch! Up here, there’s no alcohol. It’s an American thing.” He’s referring to the fact that Rubin is primarily funded by the U.S. National Science Foundation and the U.S. Department of Energy’s Office of Science, which have strict safety requirements.
On the 2,647-meter summit of Cerro Pachón, smooth air and clear skies make for some of the best “seeing” in the world.
William O’Mullane/NSF-DOE Rubin Observatory
Originally, Rubin was intended to be a dark-matter survey telescope, to search for the 85 percent of the mass of the universe that we know exists but can’t identify. In the 1970s, astronomer Vera C. Rubin pioneered a spectroscopic method to measure the speed at which stars orbit around the centers of their galaxies, revealing motion that could be explained only by the presence of a halo of invisible mass at least five times the apparent mass of the galaxies themselves. Dark matter can warp the space around it enough that galaxies act as lenses, bending light from even more distant galaxies as it passes around them. It’s this gravitational lensing that the Rubin observatory was designed to detect on a massive scale. But once astronomers considered what else might be possible with a survey telescope that combined enormous light-collecting ability with a wide field of view, Rubin’s science mission rapidly expanded beyond dark matter.
Trading the ability to focus on individual objects for a wide field of view that can see tens of thousands of objects at once provides a critical perspective for understanding our universe, says Ivezić. Rubin will complement other observatories like the Hubble Space Telescope and the James Webb Space Telescope. Hubble’s Wide Field Camera 3 and Webb’s Near Infrared Camera have fields of view of less than 0.05 square degrees each, equivalent to just a few percent of the size of a full moon. The upcoming Nancy Grace Roman Space Telescope will see a bit more, with a field of view of about one full moon. Rubin, by contrast, can image 9.6 square degrees at a time—about 45 full moons’ worth of sky.
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That ultrawide view offers essential context, Ivezić explains. “My wife is American, but I’m from Croatia,” he says. “Whenever we go to Croatia, she meets many people. I asked her, ‘Did you learn more about Croatia by meeting many people very superficially, or because you know me very well?’ And she said, ‘You need both. I learn a lot from you, but you could be a weirdo, so I need a control sample.’ ” Rubin is providing that control sample, so that astronomers know just how weird whatever they’re looking at in more detail might be.
Every night, the telescope will take a thousand images, one every 34 seconds. After three or four nights, it’ll have the entire southern sky covered, and then it’ll start all over again. After a decade, Rubin will have taken more than 2 million images, generated 500 petabytes of data, and visited every object it can see at least 825 times. In addition to identifying an estimated 6 million bodies in our solar system, 17 billion stars in our galaxy, and 20 billion galaxies in our universe, Rubin’s rapid cadence means that it will be able to delve into the time domain, tracking how the entire southern sky changes on an almost daily basis.
Cutting-Edge Technology Behind Rubin’s Speed
Achieving these science goals meant pushing the technical envelope on nearly every aspect of the observatory. But what drove most of the design decisions is the speed at which Rubin needs to move (3.5 degrees per second)—the phrase most commonly used by the Rubin staff is “crazy fast.”
Crazy fast movement is why the telescope looks the way it does. The squat arrangement of the mirrors and camera centralizes as much mass as possible. Rubin’s oversize supporting pier is mostly steel rather than mostly concrete so that the movement of the telescope doesn’t twist the entire pier. And then there’s the megawatt of power required to drive this whole thing, which comes from huge banks of capacitors slung under the telescope to prevent a brownout on the summit every 30 seconds all night long.
Rubin is also unique in that it utilizes the largest digital camera ever built. The size of a small car and weighing 2,800 kilograms, the LSST camera captures 3.2-gigapixel images through six swappable color filters ranging from near infrared to near ultraviolet. The camera’s focal plane consists of 189 4K-by-4K charge-coupled devices grouped into 21 “rafts.” Every CCD is backed by 16 amplifiers that each read 1 million pixels, bringing the readout time for the entire sensor down to 2 seconds flat.
While most telescopes have many different instruments, Rubin has only one: the LSST camera, which is the largest digital camera ever built.
Enrico Sacchetti
Astronomy in the Time Domain
As humans with tiny eyeballs and short lifespans who are more or less stranded on Earth, we have only the faintest idea of how dynamic our universe is. To us, the night sky seems mostly static and also mostly empty. This is emphatically not the case.
In 1995, the Hubble Space Telescope pointed at a small and deliberately unremarkable part of the sky for a cumulative six days. The resulting image, called the Hubble Deep Field, revealed about 3,000 distant galaxies in an area that represented just one twenty-four-millionth of the sky. To observatories like Hubble, and now Rubin, the sky is crammed full of so many objects that it becomes a problem. As O’Mullane puts it, “There’s almost nothing not touching something.”
One of Rubin’s biggest challenges will be deblending—identifying and then separating things like stars and galaxies that appear to overlap. This has to be done carefully by using images taken through different filters to estimate how much of the brightness of a given pixel comes from each object.
Designed to operate for the entire 10-year survey, the LSST camera is in some sense future-proof, with image quality that’s at the limit of what’s physically possible with the telescope that it’s attached to.
AURA/NSF-DOE Rubin Observatory
At first, Rubin won’t have this problem. At each location, the camera will capture one 30-second exposure before moving on. As Rubin returns to each location every three or four days, subsequent exposures will be combined in a process called coadding. In a coadded image, each pixel represents all of the data collected from that location in every previous image, which results in a much longer effective exposure time. The camera may record only a few photons from a distant galaxy in each individual image, but a few photons per image added together over 825 images yields much richer data. By the end of Rubin’s 10-year survey, the coadding process will generate images with as much detail as a typical Hubble image, but over the entire southern sky. A few lucky areas called “deep drilling fields” will receive even more attention, with each one getting a staggering 23,000 images or more.
Rubin will add every object that it detects to its catalog, and over time, the catalog will provide a baseline of the night sky, which the observatory can then use to identify changes. Some of these changes will be movement—Rubin may see an object in one place, and then spot it in a different place some time later, which is how objects like near-Earth asteroids will be detected. But the vast majority of the changes will be in brightness rather than movement.
The LSST camera’s 189 CCDs combine for a 9.6-degree field of view, about 45 times the area of the full moon.
AURA/NSF-DOE Rubin Observatory
Every image that Rubin collects will be compared with a baseline image, and any change will automatically generate a software alert within 60 seconds of when the image was taken. Rubin’s wide field of view means that there will be a lot of these alerts—on the order of 10,000 per image, or 10 million alerts per night. Other automated systems will manage the alerts. Called alert brokers, they ingest the alert streams and filter them for the scientific community. If you’re an astronomer interested in Type Ia supernovae, for example, you can subscribe to an alert broker and set up a filter so that you’ll get notified when Rubin spots one.
Many of these alerts will be triggered by variable stars, which cyclically change in brightness. Rubin is also expected to identify somewhere between 3 million and 4 million supernovae—that works out to over a thousand new supernovae for every night of observing. And the rest of the alerts? Nobody knows for sure, and that’s why the alerts have to go out so quickly, so that other telescopes can react to make deeper observations of what Rubin finds.
Managing Rubin’s Vast Data Output
After the data leaves Rubin’s camera, most of the processing will take place at the SLAC National Accelerator Laboratory in Menlo Park, Calif., over 9,000 kilometers from Cerro Pachón. It takes less than 10 seconds for an image to travel from the focal plane of the camera to SLAC, thanks to a 600-gigabit fiber connection from the summit to La Serena, and from there, a dedicated 100-gigabit line and a backup 40-gigabit line that connect to the Department of Energy’s science network in the United States. The 20 terabytes of data that Rubin will produce nightly makes this bandwidth necessary. “There’s a new image every 34 seconds,” O’Mullane tells me. “If I can’t deal with it fast enough, I start to get behind. So everything has to happen on the cadence of half a minute if I want to keep up with the data flow.”
At SLAC, each image will be calibrated and cleaned up, including the removal of satellite trails. Rubin will see a lot of satellites, but since the satellites are unlikely to appear in the same place in every image, the impact on the data is expected to be minimal when the images are coadded. The processed image is compared with a baseline image and any alerts are sent out, by which time processing of the next image has already begun.
Underneath the telescope, the cable drape (also called a “spider spindle”) allows power, data, and coolant lines to twist without tangling as the telescope moves.
Spencer Lowell
As Rubin’s catalog of objects grows, astronomers will be able to query it in all kinds of useful ways. Want every image of a particular patch of sky? No problem. All the galaxies of a certain shape? A little trickier, but sure. Looking for 10,000 objects that are similar in some dimension to 10,000 other objects? That might take a while, but it’s still possible. Astronomers can even run their own code on the raw data.
“Pretty much everyone in the astronomy community wants something from Rubin,” O’Mullane explains, “and so they want to make sure that we’re treating the data the right way. All of our code is public. It’s on GitHub. You can see what we’re doing, and if you’ve got a better solution, we’ll take it.”
One better solution may involve AI. “I think as a community we’re struggling with how we do this,” says O’Mullane. “But it’s probably something we ought to do—curating the data in such a way that it’s consumable by machine learning, providing foundation models, that sort of thing.”
The data management system is arguably as much of a critical component of the Rubin observatory as the telescope itself. While most telescopes make targeted observations that get distributed to only a few astronomers at a time, Rubin will make its data available to everyone within just a few days, which is a completely different way of doing astronomy. “We’ve essentially promised that we will take every image of everything that everyone has ever wanted to see,” explains Kevin Reil, Rubin observatory scientist. “If there’s data to be collected, we will try to collect it. And if you’re an astronomer somewhere, and you want an image of something, within three or four days we’ll give you one. It’s a colossal challenge to deliver something on this scale.”
Rubin creates color images by combining a series of exposures captured through different color filters. There are six of these filters, five of which can be loaded at a time into the automatic filter changer inside the camera.
SLAC National Accelerator Laboratory
The more time I spend on the summit, the more I start to think that the science that we know Rubin will accomplish may be the least interesting part of its mission. And despite their best efforts, I get the sense that everyone I talk to is wildly understating the impact it will have on astronomy. The sheer volume of objects, the time domain, the 10 years of coadded data—what new science will all of that reveal? Astronomers have no idea, because we’ve never looked at the universe in this way before. To me, that’s the most fascinating part of what’s about to happen.
Reil agrees. “You’ve been here,” he says. “You’ve seen what we’re doing. It’s a paradigm shift, a whole new way of doing things. It’s still a telescope and a camera, but we’re changing the world of astronomy. I don’t know how to capture—I mean, it’s the people, the intensity, the awesomeness of it. I want the world to understand the beauty of it all.”
The Intersection of Science and Engineering
Because nobody has built an observatory like Rubin before, there are a lot of things that aren’t working exactly as they should, and a few things that aren’t working at all. The most obvious of these is the dome. The capacitors that drive it blew a fuse the day before I arrived, and the electricians are off the summit for the weekend. The dome shutter can’t open either. Everyone I talk to takes this sort of thing in stride—they have to, because they’ve been troubleshooting issues like these for years.
I sit down with Yousuke Utsumi, a camera operations scientist who exudes the mixture of excitement and exhaustion that I’m getting used to seeing in the younger staff. “Today is amazingly quiet,” he tells me. “I’m happy about that. But I’m also really tired. I just want to sleep.”
Just yesterday, Utsumi says, they managed to finally solve a problem that the camera team had been struggling with for weeks—an intermittent fault in the camera cooling system that only seemed to happen when the telescope was moving. This was potentially a very serious problem, and Utsumi’s phone would alert him every time the fault occurred, over and over again in the middle of the night. The fault was finally traced to a cable within the telescope’s structure that used pins that were slightly too small, leading to a loose connection.
Utsumi’s contract started in 2017 and was supposed to last three years, but he’s still here. “I wanted to see first photon,” he says. “I’m an astronomer. I’ve been working on this camera so that it can observe the universe. And I want to see that light, from those photons from distant galaxies.” This is something I’ve also been thinking about—those lonely photons traveling through space for billions of years, and within the coming days, a lucky few of them will land on the sensors Utsumi has been tending, and we’ll get to see them. He nods, smiling. “I don’t want to lose one, you know?”
A doughnut-shaped screen inside the dome is used to create a uniform light source to calibrate the LSST camera. The 3.2 billion pixels of the camera sensor don’t all respond to light identically, and the calibration system provides the data necessary to compensate for these slight variations.
William O’Mullane/NSF-DOE Rubin Observatory
Rubin’s commissioning scientists have a unique role, working at the intersection of science and engineering to turn a bunch of custom parts into a functioning science instrument. Commissioning scientist Marina Pavlovic is a postdoc from Serbia with a background in the formation of supermassive black holes created by merging galaxies. “I came here last year as a volunteer,” she tells me. “My plan was to stay for three months, and 11 months later I’m a commissioning scientist. It’s crazy!”
The camera takes images through one of six color filters, five of which can be loaded into the filter changer at a time, making occasional filter swaps necessary.
Enrico Sacchetti
Pavlovic’s job is to help diagnose and troubleshoot whatever isn’t working quite right. And since most things aren’t working quite right, she’s been very busy. “I love when things need to be fixed because I am learning about the system more and more every time there’s a problem—every day is a new experience here.”
I ask her what she’ll do next, once Rubin is up and running. “If you love commissioning instruments, that is something that you can do for the rest of your life, because there are always going to be new instruments,” she says.
Before that happens, though, Pavlovic has to survive the next few weeks of going on sky. “It’s going to be so emotional. It’s going to be the beginning of a new era in astronomy, and knowing that you did it, that you made it happen, at least a tiny percent of it, that will be a priceless moment.”
“I had to learn how to calm down to do this job,” she admits, “because sometimes I get too excited about things and I cannot sleep after that. But it’s okay. I started doing yoga, and it’s working.”
From First Photon to First Light
My stay on the summit comes to an end on 14 April, just a day before first photon, so as soon as I get home I check in with some of the engineers and astronomers that I met to see how things went. Guillem Megias Homar manages the adaptive optics system—232 actuators that flex the surfaces of the telescope’s three mirrors a few micrometers at a time to bring the image into perfect focus. Currently working on his Ph.D., he was born in 1997, one year after the Rubin project started.
First photon, for him, went like this: “I was in the control room, sitting next to the camera team. We have a microphone on the camera, so that we can hear when the shutter is moving. And we hear the first click. And then all of a sudden, the image shows up on the screens in the control room, and it was just an explosion of emotions. All that we have been fighting for is finally a reality. We are on sky!” There were toasts (with sparkling apple juice, of course), and enough speeches that Megias Homar started to get impatient: “I was like, when can we start working? But it was only an hour, and then everything became much more quiet.”
Another newly released image showing a small section of the Rubin Observatory’s total view of the Virgo cluster of galaxies. Visible are bright stars in the Milky Way galaxy shining in the foreground, and many distant galaxies in the background.
NSF-DOE Rubin Observatory
“It was satisfying to see that everything that we’d been building was finally working,” Victor Krabbendam, project manager for Rubin construction, tells me a few weeks later. “But some of us have been at this for so long that first photon became just one of many firsts.” Krabbendam has been with the observatory full-time for the last 21 years. “And the very moment you succeed with one thing, it’s time to be doing the next thing.”
Commissioning scientist Marina Pavlovic watches Rubin’s first photon image appear on a monitor in the observatory’s control room on 15 April 2025.
Rubin Observatory/NOIRLab/SLAC/NSF/DOE/AURA/W. O'Mullane and R. Gill
Since first photon, Rubin has been undergoing calibrations, collecting data for the first images that it’s now sharing with the world, and preparing to scale up to begin its survey. Operations will soon become routine, the commissioning scientists will move on, and eventually, Rubin will largely run itself, with just a few people at the observatory most nights.
But for astronomers, the next 10 years will be anything but routine. “It’s going to be wildly different,” says Krabbendam. “Rubin will feed generations of scientists with trillions of data points of billions of objects. Explore the data. Harvest it. Develop your idea, see if it’s there. It’s going to be phenomenal.”
