In a brightly lit clean room at the US Department of Energy’s SLAC National Accelerator Laboratory, engineers are building a car-sized digital camera for the Large Synoptic Survey Telescope. When it’s ready, LSST will image almost all of the sky visible from its vantage point on a Chilean mountain, Cerro Pachón, every few nights for a decade to make an astronomical “movie” of unprecedented proportions.
The electronics and sensors crammed together inside the camera heat up as electricity runs through them. But heat is the enemy of astronomical observation. A warm sensor will sabotage its own observations by behaving as if it senses light where there is none. As anyone who has ever heard their laptop fan working overtime before the computer crashed may know, heat can also cause electronics to stop working.
To keep the camera cold enough, the engineers needed to create a customized cryogenic refrigeration system. They eventually made a system of eight refrigeration circuits—two for the electronics and six for the sensors.
Each of these systems works similarly to a kitchen refrigerator, in which a fluid cryogenic refrigerant carries heat away from the object or area it’s supposed to cool. Networks of tubes carry the refrigerant into and out of the camera.
“There’s a huge number of subsystems,” says Tim Bond, head of the LSST Camera Integration and Test team at SLAC. “All of those subsystems have to present their products. And all those products have to be assembled and tested and work in the final finished full product.”
The LSST Camera has a higher cryogenic heat load than previous CCD telescope cameras due to its large size (634 mm diameter focal plane, 3.2 Gigapixels) and its close-coupled front-end electronics operating at low temperature inside the cryostat. Various refrigeration technologies are considered for this telescope/camera environment. MMR-Technology’s Mixed Refrigerant technology was eventually chosen. The system is based on a cluster of Joule-Thomson refrigerators running a special blend of mixed refrigerants. The first cryogenic mix of refrigerants was developed in 2011 with improvements and refinement continuing until 2014.
Learn more about the cryogenics behind LSST: https://www.lsst.org/sites/default/files/documents/RefrigerationPlenarySchindler.pdf