Revived Technology Used to Count Individual Photons from Distant Galaxies

Adapted from a release by Fermi National Accelerator Laboratory.

Using an instrument on the 4.1-meter Southern Astrophysical Research (SOAR) Telescope, researchers obtained the first astronomical spectrum using skipper charge-coupled devices (CCDs). Skipper CCDs can get down to very low noise levels, which helps astronomers see distant galaxies.

“We had previously developed skipper CCDs for dark matter detection, and this is the first successful transition of that technology to image faint astronomical objects,” said Steve Holland, a senior engineer in the Physics Division at the Department of Energy’s Lawrence Berkeley National Laboratory. The four science-grade skipper CCDs deployed at SOAR were designed and processed at Berkeley Lab, leveraging the lab’s extensive expertise in CCD technology. This includes the development of “red-sensitive CCDs” for the Dark Energy Camera, Dark Energy Spectroscopic Instrument, and skipper-CCD designs used in dark matter experiments.

On March 31 and April 9, researchers used skipper CCDs to collect astronomical spectra from a galaxy cluster, two distant quasars, a galaxy with bright emission lines, and a star that is potentially associated with a dark-matter-dominated ultra-faint galaxy. In a first for astrophysical CCD observations, they achieved sub-electron readout noise and counted individual photons at optical wavelengths. The results were presented on June 17 at the Society of Photo-Optical Instrumentation Engineers (SPIE) Astronomical Telescopes + Instrumentation meeting in Japan.

“This is a major milestone for skipper-CCD technology,” said Alex Drlica-Wagner, a cosmologist at the DOE’s Fermi National Accelerator Laboratory and associate professor at University of Chicago who led the project. “It helps to retire the perceived risks for using this technology in the future, which is vitally important for future DOE cosmology projects.”

This is an important achievement for a project conceived and initiated through the Laboratory Directed Research and Development program at Fermilab in collaboration with NFS’s NOIRLab detector group. LDRD is a national program sponsored by the DOE that allows national laboratories to internally fund research and development projects that explore new ideas or concepts.

CCDs were invented in the United States in 1969, and forty years later scientists were awarded the Nobel Prize in Physics for their achievement. The devices are two-dimensional arrays of light-sensitive pixels that convert incoming photons into electrons. Conventional CCDs are the image sensors first used in digital cameras, and they remain the standard for many scientific imaging applications, such as astronomy, though their precision is limited by electronic noise.

Cosmologists seek to understand the mysterious natures of dark matter and dark energy by studying the distributions of stars and galaxies. To do this, they need advanced technology that can see fainter, more distant astronomical objects with as little noise as possible.

Existing CCD technology can make these measurements but take a long time or are less efficient. So, astrophysicists must either increase the signal – i.e., by investing more time on the world’s largest telescopes – or decrease the electronic noise.

Berkeley Lab’s Steve Holland designed the skipper CCDs used at SOAR for astronomical observations. After the CCDs were fabricated at Teledyne DALSA in Canada, they were processed at Berkeley Lab’s Microsystems Laboratory. The CCDs were “back illuminated,” which makes them highly sensitive to light. (Credit: Steve Holland/Berkeley Lab)

Skipper CCDs were introduced in 1990 to reduce electronic noise to levels that allow the measurement of individual photons. They do this by taking multiple measurements of interesting pixels and skipping the rest. This strategy enables skipper CCDs to increase the precision of measurements in interesting regions of the image while reducing total readout time.

In 2017, scientists pioneered the use of skipper CCDs for dark matter experiments such as SENSEI and OSCURA, but today’s presentation showed the first time the technology was used to observe the night sky and collect astronomical data.

“What’s incredible is that these photons traveled to our detectors from objects billions of light-years away, and we could measure each one individually,” said Edgar Marrufo Villalpando, a physics PhD candidate at the University of Chicago and a Fermilab DOE Graduate Instrumentation Research Award Fellow who presented the results.

Researchers are analyzing data from these first observations, which used the SOAR Integral Field Unit Spectrograph (SIFS), an instrument built by the National Astrophysical Laboratory. The next scheduled run for the skipper-CCD instrument on the SOAR Telescope is in July 2024.

“Many decades have passed since the skipper was born, so I was surprised to see the technology come to life again many decades later,” said Jim Janesick, inventor of the skipper CCD and a distinguished engineer at SRI International, a research institute based in California. “The noise results are amazing! I fell off my seat when I saw the very clean sub-electron noise data.”

With the first successful demonstration of skipper-CCD technology for astrophysics, scientists are already working to improve it. The next generation of skipper CCDs, developed by Berkeley Lab and Fermilab, is 16 times faster than current devices. Prototypes of the faster CCDs called “Multiple-Amplifier Sensing (MAS)” CCDs have been successfully tested as part of an LDRD led by Julien Guy, a physicist at Berkeley Lab.

“Jim Janesick was extremely helpful during the early days of our CCD development at Berkeley Lab, and it’s exciting to see Jim’s skipper-CCD invention ‘revived’ and merged with the fully depleted, red-sensitive CCDs,” said Holland. Berkeley Lab’s CCD effort formally began with an LDRD in 1995 and was led by Saul Perlmutter, who would later win the Nobel Prize in Physics.

The next generation of skipper CCDs has been identified for use in future DOE cosmology efforts, such as the spectroscopic experiments DESI-II and Spec-S5 recommended by the recent U.S. particle physics planning process. In addition, NASA is considering skipper CCDs for the forthcoming Habitable Worlds Observatory that will attempt to detect Earth-like planets around Sun-like stars.

“I’m looking forward to seeing where these detectors might end up,” said Marrufo Villalpando, who joined the program in 2019. “People are using them for amazing things all over; their utility ranges from particle physics to cosmology. It’s a very versatile and useful technology.”

The project was a close collaboration between physicists, astronomers, and engineers at Berkeley Lab, Fermilab, UChicago, the National Science Foundation’s NOIRLab, and the National Astrophysical Laboratory of Brazil.

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