The LSST camera at the Rubin Observatory will take extremely detailed images of the night sky from the top of a mountain in Chile. Down below the mountain, high-speed computers will send data around the world. What happens between them?

When the Vera C. Rubin Observatory begins imaging the night sky in a few years, its central 3,200-megapixel camera, the Legacy Survey of Space and Time, will provide vast amounts of data useful to everyone from cosmologists to people tracking asteroids that may hit The Earth.

You may have already read about how the Ruby Observatory’s Simonyi Survey will collect light from the universe and illuminate it on the LSST camera of the Ministry of Energy, how researchers will manage the data coming from the camera, and countless things to try. we learn about the universe around us.

What you haven’t read about is how researchers will get this mountain from very detailed photos from the back of the world’s largest digital camera, down the fiber optic cables and into computers that will transmit them from Cerro Pachón in Chile and abroad. Earth globe. .

Greg Tyrer, a scientist with the US Department of Energy’s National SLAC Accelerator Laboratory, is in charge of Rubin’s data collection system, which handles this basic process. Here he takes us through some of the key steps.

Initial steps of the Rubin Observatory data system

Initial steps of the Rubin Observatory data system. Credit: Greg Stewart / SLAC National Accelerator Laboratory

The data acquisition system starts right in the back of the focal plane, a set of 189 digital sensors used to capture images in the night sky, plus several more used to arrange the camera when taking images. 71 boards remove the raw pixels from the sensors and prepare them for the next step.

Two things have to happen at this point. First, the data must come out of a cryostat, a high-vacuum, low-temperature, and, according to Tyrer, “crowded” cavity that houses the focal plane and surrounding electronics. Second, the data must be converted into optical signals for the fibers that go to the base of the camera.

Because there is so little space inside the cryostat, Tyrer and his team decide to combine the steps: Electrical signals first enter the boards, which penetrate the back of the cryostat. These boards convert the data into optical signals that are fed into optical cables just outside the cryostat.

Why fiber optics? The data inevitably fades into noise if you go far enough on a signal cable, and the cable here must be about 150 meters or 500 feet long to get from the top of the telescope to the base. The problem is compounded by a data rate of three gigabits per second, about a hundred times faster than standard internet; low power at the heat reduction source near the sensors of the digital camera; and mechanical constraints, such as narrow bends that require cable connections where more signal is lost. Tyrer says copper conductors designed for electrical signals cannot transmit data fast enough over long distances, and even if they can, they are too large and heavy to meet the system’s mechanical requirements.

Next steps Rubin Observatory Data System

The latest steps in the Rubin Credit Observatory data system: Greg Stewart / National Accelerator Laboratory SLAC

After the signal leaves the camera, it is fed to 14 computer boards developed in SLAC as part of a general-purpose data collection system. Each board is equipped with eight on-board processing modules and 10 gigabit Ethernet switches per second, which connect the boards together. (Each board also converts optical signals back to electrical.) Three of these boards read data from the camera and prepare it for sending down the mountain to the data center in the United States at SLAC and another in Europe. Three more emulate the camera itself – in essence, they allow researchers working on the project to practice data acquisition, diagnostics, and so on when the camera itself is not available, Tyrer said.

The last eight boards serve a crucial but easily overlooked goal. “There’s a cable that runs down the mountain from the top to La Serena, where it can enter the long-distance network to data facilities in the United States and Europe,” Tyrer said. “If this cable is broken for any reason, we can buffer data for up to three days to allow the telescope to continue working during the repair.”

From the base of the telescope there is that last foot down the mountain and then the data collection is complete. It’s time for the data to come out – but you can read about it here, hereand here.

The Vera C. Rubin Observatory is a federal project co-funded by the National Science Foundation and the Ministry of Energy of Science, with funding for early construction from private donations through the LSST Corporation. The NSF-funded LSST (now Rubin Observatory) design office has been set up as an operations center under the auspices of the Association of Universities for Astronomy Research (AURA). DOE-funded efforts to build an LSST camera at the Rubin Observatory (LSSTCam) are managed by SLAC.


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