Credit: Rubin Observatory/NOIRLab/SLAC/AURAAt first glance, the Vera C. Rubin Observatory’s newest image looks like a dense field of stars.
“The COSMOS deep image is just the beginning for Rubin in this region,” said Bob Blum, director of Rubin Observatory at NSF NOIRLab.
From a Static Atlas to an Animated UniverseThe galaxies in the COSMOS field won’t visibly drift across Rubin’s images.
“The COSMOS field is a very important one for LSST science,” said Phil Marshall, Deputy Director of Rubin Observatory at SLAC.
Over its 10-year survey, Rubin is expected to detect roughly 20 billion galaxies and 17 billion stars, building a dataset containing trillions of measurements.
COSMOS field and its surroundings. Credit: Rubin Observatory/NOIRLab/SLAC/AURA
At first glance, the Vera C. Rubin Observatory’s newest image looks like a dense field of stars. Look closer, and the scale becomes almost absurd: the image contains more than half a million galaxies.
The view covers the COSMOS field, one of the most intensively studied patches of sky in astronomy. But Rubin is not just adding another deep portrait. By returning to the same field hundreds of times, it will turn this familiar snapshot into a record of supernovae, flaring black holes and other cosmic events unfolding in real time.
Same Sky, Different Telescope
For astronomers, this is familiar territory. But it’s seen through an entirely new instrument.
The COSMOS field is valuable for a simple reason: relatively little gets in the way. It lies well away from the crowded plane of the Milky Way, so fewer foreground stars and less galactic dust block the view. That gives astronomers a comparatively clear window into the distant universe.
Rubin’s sensitivity is so great, however, that even here it picks up faint wisps of dust belonging to our own galaxy. Behind them lie galaxies at an enormous range of distances and cosmic ages, including some whose light has crossed the universe for roughly 12 billion years before reaching the telescope.
Astronomers have studied this patch of sky for more than two decades. COSMOS began as a Hubble Space Telescope program in 2002, with Hubble observations starting in October 2003. It later grew into a vast collaboration involving the James Webb Space Telescope, XMM-Newton, Chandra, Spitzer, Subaru, the Very Large Array and numerous other observatories. Together, they have examined the field at wavelengths ranging from radio waves to X-rays.
The result is one of the best-documented regions of the distant universe — a multiwavelength reference library built over more than two decades.
“The COSMOS deep image is just the beginning for Rubin in this region,” said Bob Blum, director of Rubin Observatory at NSF NOIRLab. “Repeated visits to the field over the next few years will demonstrate the power of our survey design for discovery by providing our science community with a huge number of transient and variable objects like supernovae and other explosive transients for follow-up and detailed study,”
Rubin will return to the field repeatedly, allowing astronomers to identify supernovae, flaring black holes and other objects that brighten, fade or appear without warning.
The World’s Largest Digital Camera
At the heart of the observatory is the LSST Camera, a 3.2-gigapixel instrument mounted on the 8.4-metre Simonyi Survey Telescope. It is the largest digital camera ever built for astronomy.
Its focal plane contains 189 science CCD sensors (microchips that convert light to signal) arranged into 21 rafts. The sensors are cooled to around -100°C to reduce thermal noise and other unwanted signals that could interfere with observations of faint objects.
With each exposure, the camera captures 9.6 square degrees of sky — an area equivalent to roughly 45 full Moons.
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The new COSMOS view was not captured in a single snap. It is a deep coadded image made by carefully aligning and combining numerous LSST Camera observations collected between April 2025 and January 2026.
From a Static Atlas to an Animated Universe
The galaxies in the COSMOS field won’t visibly drift across Rubin’s images. The action will happen inside and around them.
A star may explode as a supernova. A black hole feeding on surrounding material may suddenly flare. A variable star may pulse, while an asteroid crosses the foreground. By repeatedly imaging the same field, Rubin can catch these changes and measure how they develop.
“The COSMOS field is a very important one for LSST science,” said Phil Marshall, Deputy Director of Rubin Observatory at SLAC. “Its wealth of prior observations, and its repeated targeting both during commissioning and as one of the LSST’s deep fields, will make it very valuable as a testing ground for scientists as they get ready to take on the survey data.”
Later phases, expected toward the end of 2026, will add individual-visit and difference images, which subtract a reference image from a new exposure to reveal what has changed. Rubin is already using difference imaging to generate scientific alerts. The system is expected eventually to issue around seven million alerts per night, directing astronomers to supernovae, asteroids, active black holes and other changing objects.
Over its 10-year survey, Rubin is expected to detect roughly 20 billion galaxies and 17 billion stars, building a dataset containing trillions of measurements. Its combination of sky coverage, depth and repeated observations will give astronomers an unusually detailed record of the southern sky.
