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Powerful Data Acquisition System To Process Space Data Obtained by the Largest Digital Camera on Earth

The Rubin Observatory’s LSST camera will capture extremely detailed images of the night sky from a mountain in Chile. Below the mountain, high-speed computers will send the data out to the world. What happens in between?

When the Vera C. Rubin Observatory starts photographing the night sky in a few years, its centerpiece will be Legacy Survey of Space and Time camera with 3,200 megapixels will provide a vast amount of data useful for everyone from cosmologists to people tracking asteroids that could hit Earth.

You may have already read about how the Ruby Observatory’s Simonyi Survey Telescope will collect light from the Universe and shine it down to Earth of the Department of Energy LSST Camera, how researchers manage the data coming from the camera and the myriad of things they will try to learn about the universe around us.

What you haven’t read is how researchers transfer this mountain of highly detailed photos from the back of the world’s largest digital camera via fiber optic cables into computers, which they broadcast from Cerro Pachón in Chile around the world.

Gregg Thayer, a US Department of Energy scientist SLAC National Accelerator Laboratory, is the person responsible for Rubin’s data collection system that handles this essential process. Here he walks us through some of the most important steps.

Ruby Observatory Data System Getting Started

Getting Started with the Rubin Observatory Data System Credit: Greg Stewart/SLAC National Accelerator Laboratory

The data acquisition system begins just behind the focal plane, a composite of 189 digital sensors used to capture night sky images, plus several others used to align the camera when capturing images. 71 boards take the raw pixels from the sensors and prepare them for the next step.

At this point two things need to happen. First, the data has to come out of the cryostat, a high-vacuum, low-temperature, and, as Thayer says, “crammed” cavity that houses the focal plane and surrounding electronics. Second, the data must be converted into optical signals for the fibers that lead to the base of the camera.

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

Why fiberglass? Data inevitably turns into noise if you go far enough along a signal cable, and the cable has to be long here — about 150 meters, or 500 feet, to make it from the top of the telescope to the base. The problem is compounded by a data rate of three gigabits per second, about 100 times faster than standard Internet; low power consumption at source to reduce heat near digital camera sensors; and mechanical limitations, such as B. Tight bends that require cable connections where more signal is lost. Thayer says that copper wires, which are designed for electrical signals, can’t carry data fast enough over the required distances, and even if they could, they’re too large and heavy to handle the system’s mechanical requirements.

Subsequent steps Rubin Observatory Data System

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

Once the signal comes down from the camera, it is fed into 14 computer boards developed at SLAC as part of a multipurpose data acquisition system. Each board comes with eight onboard processing modules and 10 gigabit-per-second Ethernet switches that connect the boards together. (Each board also converts the optical signals back to electrical.) Three of these boards read the data from the camera and prepare it to be sent down the mountain and to the US data facility at SLAC and another in Europe. Three others emulate the camera itself — essentially allowing researchers working on the project to practice collecting data, performing diagnostics, and so on when the camera itself isn’t available, Thayer says.

The last eight boards serve a crucial but easily overlooked purpose. “There’s a cable that goes down the mountain from the summit to La Serena, where it can go through the long-haul network to the US and European data facilities,” Thayer says. “If this cable is interrupted for any reason, we can cache up to three days of data so the telescope can continue to operate while the repair is being made.”

From the base of the telescope, there is a final descent down the mountain, and then the data collection is complete. It’s about time the data got out into the world – but you can read about that here, hereand here.

The Vera C. Rubin Observatory is a federal project funded jointly by the National Science Foundation and the Department of Energy Office of Science, with early construction funding coming from private donations through the LSST Corporation. The NSF-funded LSST project office (now Rubin Observatory) for construction was established as an operations center under the direction of the Association of Universities for Research in Astronomy (AURA). The DOE-funded effort to build the Ruby Observatory’s LSST Camera (LSSTCam) is managed by SLAC.



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