On Monday 29 April, the Compact High Energy Camera (CHEC) prototype camera, CHEC-S, was installed on the ASTRI-Horn telescope (left), a prototype Small-Sized Telescope (SST) for CTA. The following day, the camera was turned on and achieved first light, recording thousands of Cherenkov events in the first evening of observations. No re-alignment of the telescope optics was required following the mounting of CHEC. Images appear clear and in-focus with the PSF of the telescope well-matched to the camera pixel size. A selection of these events can be seen below.
The observations took place at the astronomical site of Serra La Nave (Mount Etna) in Sicily managed by INAF-Catania and involved both CHEC and ASTRI team members from DESY, INAF, University of Leicester, Liverpool University, Max-Planck-Institut für Kernphysik (MPIK) and the University of Oxford. The team remained on site for two weeks to prove the viability of using CHEC with ASTRI – a goal that was readily met. Beyond capturing Cherenkov images from cosmic rays, the team briefly observed several gamma-ray sources, commissioned the internal camera calibration system, took data to verify the camera pointing system and completed work on reading out trigger patterns for each raw event.[/vc_column_text][vc_column_text]
This achievement comes not long after the ASTRI-Horn telescope became the first Cherenkov telescope in a dual-mirror configuration to detect the Crab Nebula at TeV energies using the ASTRI prototype Cherenkov camera. The CHEC is an alternative Cherenkov camera compatible with both SST dual-mirror telescopes, GCT and ASTRI. The latest prototype, CHEC-S (right) consists of 2048 silicon photo-multiplier pixels forming approximately a 9o x 9o field of view when installed on ASTRI-Horn.
The CHEC is unique as an SST dual-mirror camera in its ability to capture Cherenkov light not as fixed images, but as movies consisting of hundreds of frames each lasting one billionth of a second. This can be seen clearly in the selection of Cherenkov events shown in the animated gif below. On the left, the time development of individual showers can be seen as they sweep across the camera, whilst, on the right, the resulting extracted charge for each pixel for the corresponding images is shown. The majority of these images result from showers initiated by cosmic rays of several hundred TeV landing several hundred metres from the telescope.
Two things are immediately clear from such images. First, looking at the right, there is an ambiguity in the fixed image – it is not easy to tell from which direction it originated. Think of a perfectly symmetric train – how can you tell which end is the front? The answer is that you need to see it move. The same is true here, and this ambiguity is removed by the additional timing information evident in the images to the left – we can now tell without question from which direction the Cherenkov light came. Second, each of these images lasts 128 nanoseconds. That allows Cherenkov light to propagate across the full camera without truncation, something that has not been possible with previous generations of Cherenkov cameras.
“The integration of CHEC-S on ASTRI has been extremely smooth. We have proved that the teams can work together efficiently and that CHEC works well on an ASTRI-style dual-mirror telescope,” said Richard White, Group Leader at MPIK and coordinator of the CHEC project. “We see clean, crisp Cherenkov images swimming across the camera, and results look to be almost exactly as expected from Monte Carlo simulations. Both teams have worked hard to make this a reality, and I am extremely grateful for their efforts.”
A second campaign is planned for mid-June, when the CHEC and ASTRI teams will attempt moon light observations in an important step towards verifying some of the most stringent CTA requirements. In the meantime, analysis is underway on the wealth of data collected so far. Beyond this, an iteration of CHEC is planned to incorporate the latest in SiPM technology.
Three classes of telescope are required to cover the full CTA very-high energy range (20 GeV to 300 TeV): Medium-Sized Telescopes (12 m diameter reflector) will cover CTA’s core energy range (100 GeV to 10 TeV) while the Large-Sized Telescopes (23 m) and Small-Sized Telescopes (4 m) are planned to extend the energy range below 100 GeV and above a few TeV, respectively. The ASTRI telescope and CHEC camera are proposed SST designs being prototyped and tested for CTA’s southern hemisphere array.
The ASTRI project (http://www.brera.inaf.it/astri/) is led by the Italian National Institute of Astrophysics (INAF) with the collaboration of a number of Italian universities, the Italian National Institute of Nuclear Physics (INFN), Universidade de São Paulo in Brazil and North-West University in South Africa. The CHEC project, led by MPIK, is an international collaboration between the University of Adelaide, the University of Amsterdam, DESY Zeuthen, Durham University, the Erlangen Centre for Astroparticle Physics (ECAP), the University of Leicester, the University of Liverpool, Nagoya University, and the University of Oxford.
The SSTs will outnumber all the other telescopes with 70 planned to be spread out over several square kilometres in the southern hemisphere array. Since the showers generated by very high-energy gamma-rays (between a few TeV and 300 TeV) produce a large amount of Cherenkov light, it is sufficient to build telescopes with small mirrors to catch that light. The SSTs’ wide coverage and large number, spread over a large area, will improve CTA’s ability to detect the highest energy gamma rays.
Find more technical information on CHEC in: White, R. et al. (arXiv:1709.05799)
For more information on the ASTRI project, see: S. Scuderi et al. (https://doi.org/10.1051/epjconf/201920901001)
Contacts
Richard White – CHEC Project Coordinator
Max-Planck-Institut für Kernphysik
+49-6221-516-141
Salvatore Scuderi
INAF – ASTRI Project Manager
Salvatore.scuderi@inaf.it
+39 347 0380166