by Bryné Hadnott
In an unassuming tan building, past windswept hills and equipment from the now-defunct B-Factory particle accelerator, scientists and engineers at the SLAC National Accelerator Laboratory (SLAC) have nearly finished building the world’s largest digital camera for the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST).
Construction began in 2015 and since then, the LSST camera team set a Guinness World Record for the camera’s focal plane, a table-sized array of 189 sensors working in concert to produce the highest resolution digital image ever made: 3,200 megapixels. That’s over 1500 times the resolution of a high-definition television.
The team didn’t plan on breaking any world records, but to build a camera that can take thousands of images every night—each one over three billion pixels in size—they sort of had to.
“To achieve the goals of the observatory, we need this giant focal plane. The goal of LSST is to survey the entire Southern hemisphere sky and to do it quickly enough that we get an image of every part of the available sky in three or four nights,” says SLAC professor of particle physics and astrophysics, Aaron Roodman.
Roodman has been working on the LSST camera team since 2010 and became the project’s program lead for SLAC and the Department of Energy (DOE) at the beginning of 2022. Next spring, the team will meet a critical deadline: carefully assembling three tons of delicate optical equipment and transporting it, via a chartered 747 jumbo jet, to the mountaintop observatory in Chile.
Not so Happy Accidents
The path to building the world’s largest digital camera has, at times, been a meandering one, fraught with challenges in both fundamental physics and just plain turning of screws.
“The first challenge was the development of the charge-coupled device (CCD) image sensors which started over a decade ago. They're highly multiplexed, flat to four microns, and those requirements were extremely demanding,” says Roodman.
CCDs are incredibly sensitive detectors that exploit an essential property of semiconductors called the photoelectric effect. When photons—packets of light—strike the CCD, an electron is released. By measuring the electric charge from a row of CCD pixels, scientists can detect the light from objects in the sky that human eyes can’t see. By multiplexing, or combining multiple signals into one, the team can analyze electrical signals from many portions of each CCD at once, saving critical time for pivoting the telescope to the next target.
“The next big challenge was the focal plane, which is made up of 189 CCDs organized into twenty-one subarrays. Those were built at Brookhaven lab in Long Island and then shipped to us. We retested them, and discovered these tiny metal shavings that had gotten into the wire bonds,” explains Roodman.
It took an entire year for Roodman and the camera team to disassemble the subarrays, vacuum away the shavings, and then painstakingly reassemble each one. Through a concerted effort by both Brookhaven and SLAC, the CCDs were successfully scrubbed clean and only three out of the over 3000 charge-carrying channels were affected.
What was the biggest challenge the team faced? Fitting the camera’s many components—focal plane, cryostat, controller electronics, lenses, filters, vacuum pumps, and refrigeration systems—into a single, five and a half-foot diameter cylinder.
“It’s like a ship in a bottle. The vessel containing the CCDs is very tightly integrated and further complicated by two different thermal zones: one for the CCDs at -100° Celsius and one for the electronics at around -30° Celsius," explains Roodman."The CCDs and the controller electronics are also under vacuum, which is influenced by the way particle physics detectors work, where you try to put the digitizing elements as close to the detectors as possible."
Before the camera is ready to ship to Chile, the team will test every component of the fully integrated system, from the tangle of tubes and wires supporting the camera’s electronics to the beach-ball sized, rainbow-colored filters arranged around the camera body in an innovative way.
“They're the biggest filters ever built, we should have tried them for the Guinness book too,” says Roodman. “The filters are spaced all around the cryostat on a carousel and a special mechanism grabs each one, holds it up, and cranes it over the focal plane.”
Like a garage door sliding closed, each filter is pulled down into the path of incoming light. Only light within a range of wavelengths can pass through the filter, allowing scientists to make detailed measurements of the universe in six different colors.
“Once we assemble everything, we'll probably spend a month or two taking images,” says Roodman. “That'll let us run the refrigeration systems, the vacuum systems, and the custom data acquisition system built here at SLAC.”
Capturing images on a focal plane the size of a table will require some ingenuity on Roodman’s part. Fortunately, he’s done this before. In 2020, he crafted a “pinhole projector,” an inversion of the classic pinhole camera that can take a picture with nothing but a box with a hole in one wall.
Using a folder-sized box with a 150-micron hole and four LED lights, Roodman and the SLAC engineering team were able to project an image onto the camera’s giant focal plane. They projected a photograph of Vera Rubin, a print of a Flammarion engraving, and surprisingly, a head of Romanesco broccoli.
“That was my idea,” chuckles Roodman. “I wanted a good publicity image and our normal test images are boring. It was going to be hard to focus because the focal plane is so big and, since I have a sense of the absurd, taking a picture of broccoli sounded cool to me.”
While taking pictures of broccoli might seem like just a fun diversion, the information from the test images’ orientation and viewing geometry will be invaluable to completing LSST’s camera, allowing scientists to take an unprecedentedly deep ten-year movie of the cosmos.