CTA (Cherenkov Telescope Array) is a project for the next generation of the ground-based gamma-ray observatory. CTA will cover a wide energy range, 20 GeV and 300 TeV, by 3 types of the telescopes whose diameters are different. The Large-Sized Telescopes (LSTs) of the CTA are designed to cover the lowest energy range. We are working on mirrors for 4 LSTs which will be built at La Palma, Spain. The parabolic primary mirror of CTA-LST is 23 m diameter and its focal length is 28 m. The primary mirror consists of 198 segmented mirrors. Total effective reflective area is about 370 m2. Each mirror has a hexagonal shape of 1.51 m side-by-side size. It has a sandwich structure which consists of aluminum honeycomb (60 mm thickness) and two glass surfaces (2.7 mm) and the total weight is about 47 kg. We used a sputtering deposition technique to coat in the surface with 5 layer. The coated layers protect the surface for long operation such as 10 years at outside with a reflectance degradation of less than 1 % a year. The reflectance of the mirror reaches about 92 % at 400 nm and its resolution is 0.5 mrad in diameter that contains the 80 % light reflected by the mirror. The production technique based on cold slumping gave us the stable its production. We report on the design of the segment mirror and the production of about 950 hexagonal 2 m2 mirrors to achieve the parabolic shape of the optical system for 4 LSTs.
The Cherenkov Telescope Array1 (CTA) is the next-generation ground-based observatory for very-high-energy gamma rays. The CTA consists of three types of telescopes with different mirror areas to cover a wide energy range (20 GeV–300 TeV) with an order of magnitude higher sensitivity than the predecessors. Among those telescopes, the Large-Sized Telescope (LST) is designed to detect low-energy gamma rays between 20 GeV and a few TeV with a 23 m diameter mirror. To make the most of such a large light collection area (about 400 m2), the focal plane camera must detect as much reflected Cherenkov light as possible. We have developed each camera component to meet the CTA performance requirements for more than ten years and performed quality-control tests before installing the camera to the telescope.2, 3 The first LST (LST-1) was inaugurated in October 2018 in La Palma, Spain (Figure 1).4 After the inauguration, various calibration tests were performed to adjust hardware parameters and verify the camera performance. In parallel, we have been developing the analysis software to extract physical parameters from low-level data, taking into account some intrinsic characteristics of the switched capacitor arrays, Domino Ring Sampler version 4 (DRS4), used for sampling the waveform of a Cherenkov signal. In this contribution, we describe the hard- ware design of the LST camera in Section 2, a procedure for low-level calibration in Section 3, and the readout e of the LST camera after the hardware calibration with a dedicated analysis chain in Section 4.
Many applications in high energy astrophysics, particle physics or medical imaging demand covering a large detection area with fast photosensors sensible to near-UV light. The use of Silicon photomultipliers (SIPMs) for such applications is restricted due to their sensitivity to unwanted wavelengths and mainly due to their limited physical area, increasing the cost and readout complexity of a camera. We propose to solve these drawbacks by building a Light-Trap, a low-cost pixel consisting on a SiPM attached to a PMMA disk doped with a wavelength shifter (WLS). Light in the near-UV band absorbed by the WLS is optically trapped inside the disk volume until it reaches the SiPM. The pixel collects photons over a much larger area than standard SiPMs, while being sensitive only in a desired wavelength range, which can be selected to match the application requirements. We introduce the Light-Trap principles and discuss results from laboratory measurements and Monte Carlo simulations.
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