"Normally, telescopes are bigger than this one," Fernanda Urrutia tells me, gazing at the largest piece of machinery I have ever seen.
We are inside the dome of the newly-constructed Vera C. Rubin Observatory, and Urrutia, a member of the observatory’s education and public outreach team, is my guide.
The telescope standing before us is built to be as small and fast-moving as possible. After all, it has a lot to accomplish over the next decade.
Here, at the summit of Cerro Pachón, observatory scientists are preparing for a 10-year survey of the Southern Hemisphere sky.
Earlier this year, the largest digital camera ever constructed was slotted into place as the last component of the observatory’s optical system.
Now, after months of tests and calibration, the camera’s first images have finally been unveiled.
First images
Diving into astronomical phenomena such as the Virgo Cluster and Lagoon Nebula, these images hint at the depth of discovery Rubin will be able to achieve by the end of the Legacy Survey of Space and Time (LSST) survey.
The survey, which is scheduled to begin later in 2025, will scan the entire southern sky every three to four nights and generate about 20 terabytes – 20,000 gigabytes – of data each night to answer questions about what is out there and how our Universe is changing.
"This is a very challenging project because our goals are very ambitious," says Aaron Roodman, deputy director of Rubin Construction and lead of the LSST Camera programme.
"We have pushed what is possible, in many different directions, to the limit to achieve those goals."
Unlike many telescopes that allocate time to scientists interested in viewing individual parts of the sky, Rubin’s survey will provide a broad view of the night sky and offer data useful for numerous areas of astronomy.
Delving into the Universe's biggest mysteries
First conceived as a telescope capable of unravelling the mystery of dark matter, Rubin’s design has since been optimised for a number of science goals.
Scientists expect the survey to yield a better understanding of both dark matter and dark energy, a more complete catalogue of the Solar System and Milky Way, and insights into moving and changing objects throughout the Universe.
"If you are building a system large enough, powerful enough, you can design a system that can address all the science goals with the same data set," says Željko Ivezić, director of Rubin Construction.
But building such a large and powerful system required a unique design, unlike the ground-based telescopes that have come before it.
When a photon, a particle of light, in Rubin’s path, reaches Earth from some distant celestial body, it will first hit its 8.4-metre (9.2 yards) primary mirror.
Then that photon will bounce to a smaller secondary mirror, and an even smaller tertiary mirror housed within the primary.
The primary mirror itself has travelled more than 11,000km (7,000 miles) from Arizona to the summit of Cerro Pachón, where it was coated with silver in a large, sparse area of the observatory that acts as an on-site factory.
A new eye on the sky
The telescope is a colossal mass of teal metal and intricate electronics, but combining the primary and tertiary mirrors makes Rubin smaller and faster than it otherwise would have been.
It seemingly glides between positions, allowing the LSST Camera to capture hundreds of images each night.
At the end of the survey, Ivezić estimates there will be about 1,000 pictures from each direction.
The 3,200-megapixel camera taking these pictures was assembled, component by component, at the SLAC National Accelerator Laboratory in California.
It has three lenses and a rotating set of filters to help focus photons of light and add colour before they reach one of the 189 image sensors.
Having so many image sensors means the resulting images are able to store a lot of data.
So much that it would require about 400 Ultra-HD TVs to display them fully, the project scientists say.
This image data is transported first to an in-house data centre below the camera, where whirring server racks start the images’ data processing journey.
There, images can be compared to past ones taken in the same direction, spotting changes in the night sky and sending out automatic alerts to the science community.
Roodman expects there to be anywhere between one and ten million alerts each night during the survey.
After this quick image analysis, the files are sent through a dedicated undersea fibre optic cable that goes from Cerro Pachón to the survey database at SLAC.
There, scientists can use them to work towards answering the many questions Rubin has set out to explore.
Even before data collection for the survey begins later in 2025, excitement has been building on the summit and around the world as the telescope is put through is paces.
28 countries had a hand in commissioning the telescope, and watch parties for the unveiling of the first images took place across six continents.
The scientists working on Rubin see these images as a stepping stone on the way to a better understanding of the cosmos.
"I was in the control room when we took the first image," says Alexandre Boucaud, a software engineer at the French National Centre for Scientific Research (CNRS).
"It felt like a good achievement — but five minutes later, it felt like just the beginning."
The reporting for this article was partially supported by a grant from the U.S. National Science Foundation.
