The Rubin Observatory’s LSST camera captures very detailed images of the night sky from the top of the Chilean mountain. Under the mountains, high-speed computers send data to the world. What happens in the meantime?
When the Vera C. Rubin Observatory began filming the night sky a few years later, its highlight, a 3,200-megapixel space and time legacy survey, from cosmologists to individuals tracking asteroids that could collide with Earth. It provides a large amount of data that is useful to everyone.
You may have already read about how the Rubin Observatory’s Simony Survey telescope collects light from space and illuminates it with the Energy Ministry’s LSST camera, how researchers manage data from the camera, and the myriad things they try. Maybe. To learn about the universe around us.
What you haven’t read is how researchers get a pile of very detailed pictures from behind the world’s largest digital cameras down the fiber optic cable to a computer that sends them all over the world from Selopachon in Chile. Is it done?
Gregg Thayer, a scientist at the SLAC National Accelerator Laboratory at the US Department of Energy, is responsible for Rubin’s data acquisition system, which handles this critical process. Here he will guide you through some important steps.
The data acquisition system starts just behind the focal plane. It is a complex of 189 digital sensors used to take images of the night sky and several sensors used to line up the cameras when taking images. 71 circuit boards remove raw pixels from the sensor and prepare for the next step.
At this point, two things need to happen. First, the data needs to come out of the cryostat, high vacuum, low temperature, and a “clogged” cavity that houses the focal plane and surrounding electronics. Next, we need to convert the data into a fiber optical signal that goes to the base of the camera.
Due to the very small space in the cryostat, Sayre and his team decided to combine the following steps: The electrical signal first enters the circuit board that penetrates the back of the cryostat. These circuit boards convert the data into optical signals and send them to fiber optic cables just outside the cryostat.
Why optical fiber? If you go far enough along the signal cable, the data will inevitably fade into the noise. The cable here should be long so that the distance from the top to the bottom of the telescope is about 150 meters (500 feet). This problem is exacerbated by a data rate of 3 gigabits per second, which is about 100 times faster than the standard Internet. Low power at the source to reduce heat near the digital camera sensor. Mechanical constraints that require cable interconnects that lose more signal, such as tight bends. According to Sayer, copper wires designed for electrical signals cannot transmit data fast enough over the required distance, and even if possible, are too large and heavy to meet the mechanical requirements of the system. It’s too much.
When the signal arrives from the camera, it is sent to 14 computer boards developed at SLAC as part of a general purpose data acquisition system. Each board is equipped with eight onboard processing modules that interconnect the boards and a 10 Gigabit / sec Ethernet switch. (Each board converts an optical signal into an electrical signal.) Three of these boards read data from the camera and send it down the mountain to SLAC’s US data facility and another European data facility. Get ready to do it. Three more emulate the camera itself. Basically, researchers working on a project can practice getting data, performing diagnostics, etc. when the camera itself is unavailable.
The last eight boards serve important but often overlooked purposes. “There is a cable down the mountain from the summit to La Serena, where we can connect to long-distance networks to data facilities in the United States and Europe,” Sayer says. “If the cable is disconnected for any reason, we can buffer up to 3 days of data so that the telescope can continue to operate during the repair.”
There is one last section down the mountain from the base of the telescope, which completes the acquisition of the data. It’s time for the data to be sent to the world, which you can read here, here, and here.
Vera C. Rubin Observatory is a federal project jointly funded by the National Science Foundation and the Department of Science of the Ministry of Energy, receiving initial construction funding from private donations through LSST Corporation. The NSF-funded LSST (now Rubin Astronomical Observatory) Construction Project Office has been established as an operating center under the control of the Association of Universities for Astronomy (AURA). The DOE-funded efforts to build the Rubin Observatory LSST Camera (LSSTCam) are managed by SLAC.