by Bryné Hadnott

Celebrated physicist and Rubin Observatory project scientist Steve Kahn has crisscrossed the country multiple times during his academic career—from a PhD at University of California Berkeley to Columbia University and back west to Stanford—but his biggest leap was shifting from a decades-long career in X-ray astronomy to building the world’s largest digital camera for the Legacy Survey of Space and Time (LSST, whose current status was discussed in this recent feature in March 2022).

“I actually changed my research completely by moving from Columbia to Stanford. I was doing X-ray astronomy, measuring the spectra from paradigmatic sources of particular types—black holes, neutron stars, supernova remnants,” says Kahn. “I'd never done optical astronomy, really, and I’d certainly never built anything on the ground.”

During the early 1990s, Kahn developed the first reflection grating spectrometer (RGS) to ever fly on an X-ray mission for the European Space Agency’s X-ray Multi Mirror Mission (XMM-Newton). Reflection gratings split light into a rainbow of different wavelengths using a shiny mirror covered in finely spaced grooves less than a hair’s width apart. The RGS on XMM-Newton was capable of measuring wavelengths to an accuracy of five milliangstroms, a ten-trillionth of a meter.

Kahn’s prominence in X-ray astronomy led to a position on the steering committee for the 2000 Astronomy Decadal Survey. It was there that he was introduced to a novel proposal from Bell Labs scientist Tony Tyson (now a professor at UC Davis), for the then-named Large-aperture Synoptic Survey Telescope (now called the Charles Simonyi Telescope at the Vera Rubin Observatory). The goal of the project was unprecedented: survey the entire Southern sky every week to uncover the previously hidden structures unique to our dark energy-dominated Universe.

“The discovery of the acceleration of the Universe in ‘98 was such a startling thing. Then, WMAP (Wilkinson Microwave Anisotropy Probe showed that the Universe was flat,” says Kahn. “All of these cosmological problems just went away overnight. Prior to this discovery, the oldest stars in the Universe were older than the Universe!”

Laying the groundwork for LSST

In 2003, Kahn was asked to become the deputy director of the newly formed Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint organization that brought together researchers from Stanford University’s physics department with technologists from the SLAC National Accelerator (SLAC).

“The confluence of this new revolution in cosmology and this new way of thinking about building a telescope as an experiment with particular measurements in mind, got me thinking, ‘this is what I want to do’,” says Kahn. “Roger Blandford and I were both being interviewed for positions at KIPAC—he was a theorist, I was an experimentalist—and we both thought LSST would be a good project for SLAC.”

“I like to think I encouraged him to do it and he encouraged me,” says founding KIPAC director, Roger Blandford. “I think we both had a sense that if one of us dropped out, we were individually less likely to do it.”

In between teaching physics classes and attracting the next generation of talented astrophysicists—rising stars like current KIPAC Director Risa Wechsler, Steve Allen, and Tom Abel—to KIPAC, Kahn traveled the country with Blandford and Tyson, giving talks to curious scientists and potential donors on the significance of LSST to fundamental cosmology research.

“It's such a simple concept, but the implications are profound,” says Kahn. “I'm more of a physicist than an astronomer and the way a physicist thinks about a project like LSST is starting with what you're trying to measure. When KIPAC started, we defined it to be the ‘physics of astrophysics’ to fit in better with SLAC and the rest of the physics department.”

The study of the “physics of astrophysics” was a driving force in Kahn’s approach to designing LSST and growing KIPAC into a community of top-tier researchers in cosmology, astrophysics, and optical astronomy. After serving as KIPAC’s deputy director for more than four years, Kahn became the director of Particle Physics and Astrophysics at SLAC, where he continued to garner support for LSST from private donors—including Charles Simonyi and Bill Gates—the National Science Foundation (NSF), and the Department of Energy (DOE).

It wasn’t until 2011, after LSST was ranked at the top of the 2010 Astronomy Decadal Survey’s list of ground-based telescopes, that Kahn and the LSST team at SLAC received “Critical Decision 0” approval from the DOE. Finally, they could start construction on the eye of the telescope, a five-foot-wide, 3,200-megapixel camera able to image the deepest and faintest objects in the night sky.

“The camera was technically super challenging on almost all fronts. That's one of the reasons I liked it,” says Kahn. “Every element of it was beyond the state of the art. It's fundamentally different in architecture than all other astronomical cameras and of course, we had crises. Everything that could be broken, broke.”

The Next Big Challenge

For nearly a decade, Kahn served as the LSST Project director, a position that had him traversing repeatedly the southwestern Sonoran Desert from his home in California’s Bay Area to the National Optical Astronomy Observatory in Tucson. He was also the Camera Lead Scientist at SLAC, guiding the development of the camera's heart, a more than two-foot-wide focal plane made up of 189 individual imaging sensors. The fully integrated camera—a towering assemblage of lenses, filters, cryostats, and the focal plane—was on track for an “early completion date” in May 2022, but COVID-19 halted development everywhere from the LSST camera cleanroom at SLAC to Rubin Observatory’s construction in Chile.

“COVID was the worst thing you could imagine, not just because it was a global pandemic, but these waves came in at different times and places,” says Kahn. “I was working at home a lot and then in the middle of this I was recruited by Berkeley for a deanship.”

Starting in June, Kahn will become the new dean of the Division of Mathematical and Physical Sciences at the University of California, Berkeley’s College of Letters and Sciences. He’ll cover five different departments: mathematics, physics, astronomy, earth and planetary science, and statistics.

“This is a new kind of challenge, like building a telescope,” says Kahn on his return to UC Berkeley. “How do we change the character of universities to make then more open and inclusive?”

While Kahn won’t miss the three and a half hour commutes between his home in Berkeley to Palo Alto, he’ll continue to remain involved at Stanford as a professor emeritus and is a core member of Rubin Observatory’s Project Science Team.

“I think Roger and I joined KIPAC at the best stage. We learned from all of these tough starts and that really gave us a different perspective,” reflects Kahn. “There were a lot of interesting things that happened and most of these stories don’t ever get told. You're always trying to do something that's by definition harder—it makes it exciting.”