The debut of a new detector has many “firsts”: the first assembly, the first shift, the first light, the first detection… But if there’s one thing that makes a debut official—sort of like a detector’s birth certificate—it’s the detailed description of how the detector was built and how it performs.
And this is achieved in a new paper by members of the Cherenkov Telescope Array Consortium, published in the Journal of Astronomical Telescopes, Instruments and Systems. The paper documents the design of the camera of the prototype Schwarzschild-Couder Telescope (pSCT), a medium-sized candidate telescope for the Cherenkov Telescope Array (CTA). The paper also includes performance metrics that show its potential as a very-high-energy gamma-ray detector and that have already been used to plan an upgrade, a project which is now well underway. Very high energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects, including black holes and possibly dark matter.
The pSCT uses novel dual-mirror optics, rather than more traditional single-mirror optics, and relies on high-speed electronics to cover CTA’s middle energy range from 80 GeV to 50TeV. This camera was developed by a team spanning multiple universities and co-led by UW–Madison physics professor Justin Vandenbroucke, who has been working on this project since 2009.
The use of a secondary mirror to correct aberrations and enhance the focusing quality of Cherenkov light was proposed by Karl Schwarzschild in 1905, but it is first being used now, for imaging atmospheric Cherenkov telescopes (both the pSCT and the small-sized telescopes of CTA). This allows building a high-resolution imaging telescope with a much smaller camera using novel silicon photomultipliers, thus maintaining construction costs while yielding a telescope and camera with excellent imaging resolution.
The pSCT camera features a mechanical structure in which the photosensor modules are inserted. In addition to silicon photomultipliers, these modules include fast electronics that can trigger on incoming signals and sample them at a rate of one billion frames per second and will capture flashes of Cherenkov light created by gamma-ray interactions in Earth’s atmosphere. Multi-module trigger and readout functions are enabled by a backplane and data acquisition boards. Camera operations are supported by auxiliary systems including a cooling system, shutter, alignment system, flashers, power supplies, and a camera server.
The paper describes performance metrics for the pSCT that include the temperature dependence of the noise and telescope trigger, the method of waveform calibration, and camera software tests.
These measurements were taken with the initial 1600-pixel camera—encompassing 25 modules each containing 64 pixels—that will soon be upgraded to 11,328 pixels, increasing the field of view from 2.7 to 8 degrees, which will make the pSCT the imaging atmospheric Cherenkov telescope with the largest field of view to date.
Publishing this paper involved many firsts for Leslie Taylor, a postdoctoral scholar at UW–Madison, who joined Vandenbroucke’s lab in 2017 when his group was busy integrating and testing the camera in their Chamberlin Hall lab. The team then shipped it to the Fred Lawrence Whipple Observatory in southern Arizona and installed it on the telescope, where it is now operating next to the VERITAS telescope array.
Leslie enjoyed every minute of this hands-on work both in the lab and during multiple trips to Arizona. There, she helped with the on-site installation of the telescope and also learned to operate it. This work led her to write the operations manual that others have used since. She took the first data in January 2019 after the mirrors of the telescope were uncovered to receive what scientists call “first light,” i.e., when the first Cherenkov light from particle showers was detected. In the subsequent year, she played a leading role in commissioning the telescope and taking its first observational data. Analysis of this data by the team resulted in the announcement that the pSCT had observed the Crab Nebula. This detection of a supernova remnant, in photons a trillion times more energetic than we can see with our eyes, confirmed the pSCT as a new gamma-ray telescope.
Now, Leslie has closed a chapter by leading the paper that summarizes years of collaborative work by the full team. “Helping to put this paper together has been an amazing feat for me. It is so satisfying to see the years of hard work that I and my colleagues have put in laid out in such a clear way. I know that this paper will be a pivotal point of reference for our work going forward,” she said.
While the first data were taken, and then analyzed, Vandenbroucke’s team and their colleagues have continued working on improvements to the pSCT camera. “We are now busy upgrading the camera based on what we learned from the current camera and documented in this paper. Our goal is for the upgraded camera to be capable of detecting single Cherenkov photons, which will enable excellent gamma-ray astrophysics across a broad energy range,” explains Justin.
The pSCT team of the CTA Consortium includes more than 30 institutions across the US, Italy, Germany, Japan, and Mexico. When completed, CTA will become the most sensitive very high energy gamma-ray observatory and will be made up of small, medium, and large telescopes split between a southern array in Paranal, Chile, and a northern array on the island of La Palma, Spain. In the US, construction funding was provided through the National Science Foundation (NSF) Major Research Instrumentation Program. The pSCT operations are funded by NSF and the Smithsonian Institution. The UW–Madison group is supported by the National Science Foundation, the Office of the Vice Chancellor for Research and Graduate Education, and the Wisconsin Alumni Research Foundation.
+info Design and performance of the prototype Schwarzschild-Couder telescope camera, Colin B. Adams et al, J. of Astronomical Telescopes, Instruments, and Systems, 8(1), 014007 (2022). https://doi.org/10.1117/1.JATIS.8.1.014007
Check out this video for more background on the project and the UW team: