Design, optimization and characterization of the light concentrators of the single-mirror small size telescopes of the Cherenkov Telescope Array

Abstract

The focal-plane camera of γ-ray telescopes frequently uses light concentrators in front of light sensors. The purpose of these concentrators is to increase the effective area of the camera as well as to reduce the stray light coming at large incident angles. These light concentrators are usually based on the Winston cone design. In this contribution we present the design of an hexagonal hollow light concentrator with a lateral profile optimized using a cubic Bézier function to achieve a higher collection efficiency in the angular region of interest. The design presented here is optimized for a Davies-Cotton telescope with primary mirror of about 4 meters of diameter and focal length of 5.6 m. The described concentrators are part of an innovative camera made up of silicon-photomultipliers sensors, although a similar approach can be used for other sizes of single-mirror telescopes with different camera sensors, including photomultipliers. The challenge of our approach is to achieve a cost-effective design suitable for standard industrial productions of both the plastic concentrator substrate and the reflective coating. At the same time we maximize the optical performance. In this paper we also describe the optical set-up to measure the absolute collection efficiency of the light guides and demonstrate our good understanding of the measured data using a professional light tracing simulation.

The front-end electronics and slow control of large area SiPM for the SST-1M camera developed for the CTA experiment

“…The photodetector plane of the camera has a hexagonal shape of about 90 cm sideto-side and contains 1296 pixels, each consisting of a SiPM and a hollow light concentrator. More details on the optical design and chosen pixel size can be found in [4]. The pixel size required in order to achieve the global desired angular resolution of 0.24°at 4°off-axis is 2.4 cm.…”

The single mirror small sized telescope for the Cherenkov telescope array

“…As a consequence, the duty cycle increases enhancing the overall sensitivity of the full array in the high energy range. The camera dimensions in the focal plane (97 cm flat-to-flat for the full structure, 88 cm flat-to-flat for the photo-detection plane) are dictated by the telescope geometry, together with the CTA requirement of having a field of view above 8 • for SSTs [4]. The hexagonal-shaped enclosure (73.5 cm thick) hosts the photo-detection plane (PDP), DigiCam, the power supplies, the housekeeping and the cooling system in a structure that complies at least with the IP65 standard, and which weighs less than 200 kg.…”

“…transmittivity of cones, dark count rate and cross talk of sensors, full functional tests of the electronics and optical tests of the modules (see Refs. [4,10,13,14]) have been measured systematically in dedicated setups. An important figure of merit of the full system (optical system plus full readout chain) is the charge resolution, for which CTA dictates strict requirements on signals from 1 to 2000 photo-electrons per pixel.…”

“…The geometry of the concentrators has been optimized to reach the required 24 • cut-off angle and 23.2 mm flat-to-flat pixel size on an hexagonal surface (see Ref. [4]). The fabrication process (substrate and coating) has been optimized to enhance the reflectivity of the inner surface in the Cherenkov wavelength range.…”

“…Its design has been driven by the CTA design concept requirements: a point spread function for SSTs ≤ 0.25 • at 4 • off-axis; the telescope must focus light over 80% of the required camera field of view diameter with an rms optical time spread of less than 1.5 ns; a field of view ≥ 8 • . These requirements determine the overall geometry of the telescope [4], including the camera pixels desbribed in section 2.3. Using a Davies-Cotton design, a 4 m reflector diameter with a focal ratio of 1.4 translates into a focal length of 5.6 m and satisfies the requirements.…”