You may have already read about how the Rubin Observatory’s Simonyi Survey Telescope gathers light from the universe to shine it onto the Department of Energy’s LSST camera. Also, how researchers will handle the data from the camera and what they will try to learn about our universe.
You don’t know how researchers will extract the mountain of photos from the world’s largest digital camera and down fiber optic cables to computers that will transmit them over Cerro Pachon, Chile, and around the globe.
Gregg Thayer is a scientist at U.S. Department of Energy’s SLAC National Accelerator Laboratory. He is responsible for Rubin’s data acquisition process, which manages this vital process. He walks us through the most important steps.
The data acquisition system begins at the back, with a combination of 189 digital sensors that are used to capture night-sky images and several others used to align the camera to take images. The raw pixels are removed from the sensors by 71 circuit boards and prepared for the next step.
Two things must happen at this point. First, data must be extracted from the cryostat. This is a low-temperature, high-vacuum and jam-packed cavity that houses the focal plan and other electronics. The data must be converted to optical signals to send to the base of your camera.
Thayer and his team combined the steps because there is so little space in the cryostat. These circuit boards convert data into optical signals which are then fed into fiber optic cables outside the cryostat. Why fiber optics? Fiber optics are a great choice. Data fades to noise when it is extended along a signal cable. The cable must be approximately 150m long (or 500 feet) to reach the telescope’s base. This problem is made worse by three gigabits per second data rates, which are around one hundred times faster than standard internet. Low power at the source to lower heat near digital camera sensors. Mechanical constraints such as tight bends that require interconnecting cables where signal loss is greater. Thayer claims that copper wires, which are designed to transmit electrical signals, cannot transmit data fast enough over long distances.
Once the signal has reached the camera, it is fed into 14 computers boards that were developed by SLAC in a general-purpose data acquisition program. Each board has eight processing modules onboard and 10 gigabit per-second Ethernet switches to connect them together. Each board converts optical signals back into electrical signals. The data is read by three boards, which prepare the data to be sent to the U.S. data center at SLAC in the USA and to another European facility. Thayer states that three more boards mimic the camera and allow researchers to practice taking data, performing diagnostics, etc., when the camera is not available.
These eight boards serve an important but often overlooked function. Thayer explains, “There is a cable that runs down the mountain from La Serena to the summit. It can then connect to the long-haul network to America and European data facilities.” We can buffer upto three days of data if the cable is cut, so the telescope can continue to operate during repairs.
The final step down the mountain from the telescope’s base is the data acquisition. It’s now time to send the data out into the wild – you can read more here and hier about this , here.
The federal Vera C. Rubin Observatory project is jointly funded by the National Science Foundation, the Department of Energy Office of Science and early construction funding was received through private donations through LSST Corporation. As an operating center, the Association of Universities for Research in Astronomy established the LSST Project Office for construction. It was funded by the NSF. SLAC manages the DOE-funded effort for building the Rubin Observatory LSST Cam (LSSTCam).
