CTA prototype telescope records first light and first particle showers

Less than a week after its inauguration on January 17, 2019, the prototype Schwarzschild-Couder telescope (pSCT), telescope design proposed for the Cherenkov Telescope Array (CTA), successfully detected its first Cherenkov light on January 23 at the Fred Lawrence Whipple Observatory in Arizona. A dual-mirrored medium-sized telescope, the SCT is proposed to cover the middle of CTA’s energy range (80 GeV–50 TeV).

The week following the inauguration, the camera was operated with the mirrors uncovered for the first time in a commissioning test run. During the first few minutes of this “first light” run on the evening of January 23, cosmic-ray-induced Cherenkov air shower events were identified in the raw, uncalibrated camera data. The video below shows 50 nanoseconds of a single event in which the Cherenkov air shower development is recorded by the camera with one nanosecond resolution (time between video frames).

“In research, things usually don’t work on the first try,” says Justin Vandenbroucke, an assistant professor at UW–Madison and a leader of the pSCT camera over the last decade. “You learn to expect surprises. When we turned on the camera with the mirrors uncovered for the first time and immediately detected particle showers, I was surprised that there were no surprises!”

Professor Justin Vandenbroucke (University of Wisconsin–Madison) with the prototype Schwarzschild-Couder telescope at the Fred Lawrence Whipple Observatory on Mt. Hopkins in Arizona.  Photo by Leslie Taylor.

Cherenkov light is the result of a gamma ray or cosmic ray from an astrophysical source interacting with the Earth’s atmosphere. The flash of bluish light only lasts a few billionths of a second and is extremely faint. Gamma-ray telescopes are sensitive to these faint flashes.  The pSCT camera triggers when several neighboring pixels detect light within a few nanoseconds of one another. The camera has a modular design, with 25 modules each containing 64 pixels. The central module is not yet installed, in order to shine a laser beam along the central axis for telescope alignment, and a neighboring module was disabled during the test run.  The pixel amplitudes are raw and uncalibrated, but the early results are a major milestone for the SCT team.

The WIPAC CTA team, led by Vandenbroucke, has worked on the construction and integration of the camera for the last five years. Assembly and testing of the camera was completed in Madison in spring 2018, and UW–Madison undergraduate and graduate students have played essential roles in the project.

Graduate student Leslie Taylor was on-site the night of first light detection. She operated the telescope with Vandenbroucke and with engineer Thomas Meures connected remotely from Madison.  As soon as Taylor recorded the first events, all three were amazed and overjoyed to see––right away––shapes that looked like particle showers. Vladimir Vassiliev, pSCT PI and UCLA professor, also joined from Los Angeles and confirmed that these were Cherenkov showers, the signature of a high-energy particle interacting with the Earth’s atmosphere. For Vassiliev, who first proposed this telescope design in 2007, this was a dream come true.

As Meures explains, “It seemed way too easy that the first time we ran the telescope we recorded these nice images. It felt great that years of challenges and hard work in the lab had paid off.”

Engineer Thomas Meures (left) and PhD student Leslie Taylor (right), both from the University of Wisconsin–Madison, preparing the pSCT camera for installation onto the telescope at the Fred Lawrence Whipple Observatory.  The rear of the camera, including electronics for triggering on and recording the 1600 camera pixels at a rate of one billion frames per second, is visible.  Photo by Justin Vandenbroucke.

The SCT’s dual-mirror optical system improves on the single-mirror designs traditionally used in gamma-ray telescopes by dramatically enhancing the focusing quality of Cherenkov light over a large region of the sky and by enabling the use of compact, highly efficient photo-sensors in the telescope camera. Commissioning of the pSCT will continue in 2019, including alignment of the panels of both the primary and secondary mirrors, positioning of the camera with respect to the focal plane, and calibration of the camera data.  A project is underway to increase the number of pixels in the pSCT camera by a factor of seven in order to match the wide field of view of the mirrors.

“Working on the pSCT has been a truly collaborative process. People come to Arizona from all over the US and the world. They all work so hard to make this telescope possible. It is gratifying to see work done by so many people fit together so seamlessly,” says Taylor.

CTA will consist of 118 telescopes split between a southern array in Paranal, Chile and a northern array on the island of La Palma, Spain. Three classes of telescopes (small-, medium- and large-sized telescopes) will be used to detect gamma rays in the energy range of 20 GeV to 300 TeV with about ten times increased sensitivity compared to any current observatory. Notable for providing improved gamma-ray angular resolution and a very-high-resolution camera (11,000 pixels), the SCT is proposed for the medium-sized CTA telescopes, which are considered to be the “workhorses” of the arrays, with 15 planned for the north site and 25 for the south site.

The pSCT project is made possible by funding provided by the U.S. National Science Foundation, by the contributions of other partner U.S. and international funding agencies, and by the efforts of thirty institutions and five key industrial partners across the United States, Italy, Germany, Japan, and Mexico. Full list can be found here.

Other multimedia resources:

More images (link)

More events recorded during first light night (link)