Building SPHEREx in a Caltech Basement

Chi Nguyen remembers the moment she plugged her laptop into a large monitor in a conference room and onlookers began to cheer. She was presenting a team of scientists, engineers, and managers at NASA's Jet Propulsion Laboratory (JPL), which is managed by Caltech, with the first "aliveness test" image taken by the agency's space telescope SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer). The telescope successfully launched into space from Vandenberg Space Force Base in central California on March 11, 2025. On March 18, mission specialists instructed the telescope's lens cap to pop off; about a week later, SPHEREx snapped a picture, not of any stars in focus yet—that happened later in a momentous event called "first light"—but an image to assess the health of the detectors.

"The picture was beautiful," says Nguyen, a postdoctoral scholar at Caltech whose job was to carefully characterize the detectors. "It looked blurry, like a photo taken from a moving car. This was expected because we had yet to command the spacecraft to maintain stable pointing, but this snapshot was enough to tell us that our detectors were working properly. We breathed a sigh of relief because we knew our years of work paid off."

Nguyen is part of a small team of Caltech scientists, including post-baccalaureate and undergraduate students, who helped build and test the SPHEREx instrument, which includes the telescope, two focal-plane cameras, and readout electronics. The mission is currently up in space taking 3,600 unique images a day that capture more than 100 wavelengths of infrared light. Over the next two years, SPHEREx will scan the entire sky four times, creating 3D maps of hundreds of millions of galaxies in our universe. The mission's science goals include studying the birth of our universe, the evolution of galaxies, and interstellar ices in regions of our Milky Way galaxy where stars and planets form.

The Caltech scientists worked closely with several partners to build and test SPHEREx. BAE Systems built the telescope and spacecraft, while the Korea Astronomy and Space Science Institute (KASI) built a customized chamber for testing the instrument in a basement laboratory at Caltech. Caltech led the design and build of the instrument electronics, and Caltech's IPAC astronomy center developed the mission's data pipeline—software that processes the raw data into calibrated images and spectra (detailed analyses of the different wavelengths of light coming from an object).

Yet the most important partnership in building the instrument lay just a 15-minute drive away. Teams from Caltech's campus and JPL worked synergistically to test SPHEREx and get it ready for space flight.

"SPHEREx's success highlights the close interplay with JPL personnel, and that worked in large part due to personal relationships established over decades," says Jamie Bock, the mission's principal investigator, the Marvin L. Goldberger Professor of Physics at Caltech, and a senior research scientist at JPL.

Each group brought their expertise to the table. JPL excels at putting missions in space, while Bock's team specializes in building instruments custom designed for cosmology measurements. For example, some of the team working on SPHEREx, including Bock, also built instruments for the ongoing BICEP (Background Imaging of Cosmic Extragalactic Polarization) cosmology experiments at the south pole.

"When you are working on space missions, you can't just pick up a screwdriver and go for it," says Stephen Padin, a research professor of physics at Caltech and the SPHEREx optics team lead. "That's where the balance between the Lab and campus helps out. JPL understands how careful one must be for a space mission, because obviously you can't go to space and fix it. Meanwhile, academic groups like ours know how to design, build, and test astrophysics instruments. Combine that academic know-how with JPL expertise and great things happen."

Bock agrees, pointing out that campus and the Lab continue to look at new ways to produce lower-cost space missions. SPHEREx, he says, represents an "example of how the unique environment that leverages expertise from Caltech and JPL makes it all work in practical reality."

Not All Pixels Are Created Equally

Howard Hui, the mission's calibration scientist, says his experience with BICEP—a ground-based project he began working on with Bock as a graduate student in 2010—has been invaluable for designing and testing SPHEREx. Like SPHEREx, the BICEP experiment is designed to pick up signatures from the explosive birth of our universe, an event known as cosmic inflation, during which our universe expanded by a trillion-trillion-fold in a fraction of a second. The microwave detectors at the heart of BICEP are still making the most sensitive measurements yet for the type of cosmology it is doing.

SPHEREx is searching for a different set of signatures from the cosmic inflationary epoch. Though it uses a different type of detector than BICEP, Hui says his work on the ground-based telescope prepared him for the meticulous work a space mission like SPHEREx requires.

"We have spent the last three years trying to understand how our instrument behaves," he says. "There's no such thing as a perfect experiment. There are artifacts that come from Earth's upper atmosphere, satellites, and our instrument. To study inflation, we have to understand all of that noise really well, so we can separate it out and accurately see galaxies and other cosmic objects."

One of Hui's tasks involved characterizing the 24 million pixels that make up the SPHEREx detectors. "Every pixel has a different wavelength response that we have to carefully measure," Hui explains. "What makes the SPHEREx team unique is that we are an academic group of astrophysicists with a lot of hands-on experience."

Sapphire Window

To mimic the chill of outer space in their laboratory, the Caltech team turned to colleagues at KASI, who built them a customized SUV-sized chamber. The chamber cooled the instrument down to frosty temperatures of minus 350 degrees Fahrenheit (about minus 200 degrees Celsius). Several tests were performed inside the chamber, including adjustments of the focus of the telescope and other measurements to calibrate and characterize the instrument's detectors and its unique spectrometers—devices that split light into discrete colors.

A specialized gold-coated sapphire window allowed the team to shine an artificial star inside the chamber while reflecting ambient light away.

"The lab is glowing at infrared wavelengths," explained Phil Korngut, instrument scientist for SPHEREx, in a Caltech news story about the chamber tests. "We need to keep that nasty thermal background light from getting into the telescope because it would totally swamp the detectors."

Steve Padin working on the SPHEREx instrument in a basement lab at Caltech. He is standing in front of the specialized test chamber built by KASI, and adjusting relay mirrors to focus an artificial star inside the chamber. Sam Condon's face can be seen in the background, reflected off the chamber's sapphire window. Credit: Caltech

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Steve Padin working on the SPHEREx instrument in a basement lab at Caltech. He is standing in front of the specialized test chamber built by KASI, and adjusting relay mirrors to focus an artificial star inside the chamber. Sam Condon's face can be seen in the background, reflected off the chamber's sapphire window.

Credit: Caltech

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Hui says that he, Korngut, and Brad Moore, the mission's payload thermal engineer at JPL, traveled to Korea to develop the chamber with their KASI colleagues. "We wanted to create the same environment that SPHEREx will reside in in space," Hui says. "We were supposed to go back to Korea every few months, but then COVID happened, so it was a bit more complicated to develop across continents."

Meanwhile, JPL colleagues, who had also worked on aspects of the instrument's design, visited campus to help with testing. During this time, the telescope was brought to JPL to perform a shake test, which ensured SPHEREx's sensitive equipment would survive the bumpy ride to space aboard a rocket.

In the end, after the instrument had been carefully characterized, calibrated, and tested, it was shipped to BAE Systems in Colorado for integration with the spacecraft prior to launch.

"We can't easily calibrate the instrument in space, so we tried to understand as much of the instrument as possible in our Caltech lab. I love the lab work and love that we were fully involved with characterizing the instrument," Nguyen says. "We made it possible for SPHEREx to peer out into the cosmos and accurately detect galaxies all around us."

In addition to SPHEREx, three other NASA space missions of a similar size class, called Explorer missions, have been led by Caltech, including GALEX, NuSTAR, and the upcoming UVEX. In fact, the NuSTAR team lent their electronics experts, led by Rick Cook of Caltech, to the SPHEREx team to help build the instrument.

An academic setting also meant that students working on the missions received world-class, hands-on training. One SPHEREx team member, Sam Condon, who worked on the mission for three years after graduating from college, was inspired to take physics classes while working, so he could go to graduate school. He is now earning his PhD in physics at Stanford University.

"There's no other place like Caltech, where, as a student, you can build a space telescope in the laboratory basement," Bock says. "It's a special place where NASA and academia meet."