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Register a new userDesign of light concentrators for Cherenkov telescope observatories
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Added by
Pierre-Olivier Petrucci
Publication date
2013
fields
Physics
and research's language is
English
Authors
F. Henault
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The Cherenkov Telescope Array (CTA) will be the largest cosmic gamma ray detector ever built in the world. It will be installed at two different sites in the North and South hemispheres and should be operational for about 30 years. In order to cover the desired energy range, the CTA is composed of typically 50-100 collecting telescopes of various sizes (from 6 to 24-m diameters). Most of them are equipped with a focal plane camera consisting of 1500 to 2000 Photomultipliers (PM) equipped with light concentrating optics, whose double function is to maximize the amount of Cherenkov light detected by the photo-sensors, and to block any stray light originating from the terrestrial environment. Two different optical solutions have been designed, respectively based on a Compound Parabolic Concentrator (CPC), and on a purely dioptric concentrating lens. In this communication are described the technical specifications, optical designs and performance of the different solutions envisioned for all these light concentrators. The current status of their prototyping activities is also given.
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Reflective light concentrators with hexagonal entrance and exit apertures are frequently used at the focal plane of gamma-ray telescopes in order to reduce the size of the dead area caused by the geometries of the photodetectors, as well as to reduce the amount of stray light entering at large field angles. The focal plane of the large-sized telescopes (LSTs) of the Cherenkov Telescope Array (CTA) will also be covered by hexagonal light concentrators with an entrance diameter of 50 mm (side to side) to maximize the active area and the photon collection efficiency, enabling realization of a very low energy threshold of 20 GeV. We have developed a prototype of this LST light concentrator with an injection-molded plastic cone and a specular multilayer film. The shape of the plastic cone has been optimized with a cubic B{e}zier curve and a ray-tracing simulation. We have also developed a multilayer film with very high reflectance ($gtrsim95$%) along wide wavelength and angle coverage. The current status of the prototyping of these light concentrators is reported here.
We have developed a prototype hexagonal light concentrator for the Large-Sized Telescopes of the Cherenkov Telescope Array. To maximize the photodetection efficiency of the focal-plane camera pixels for atmospheric Cherenkov photons and to lower the energy threshold, a specular film with a very high reflectance of 92-99% has been developed to cover the inner surfaces of the light concentrators. The prototype has a relative anode sensitivity (which can be roughly regarded as collection efficiency) of about 95 to 105% at the most important angles of incidence. The design, simulation, production procedure, and performance measurements of the light-concentrator prototype are reported.
The Cherenkov Telescope Array (CTA) is an international next-generation ground-based gamma-ray observatory. CTA will be implemented as southern and northern hemisphere arrays of tens of small, medium and large-sized imaging Cherenkov telescopes with the goal of improving the sensitivity over the current-generation experiments by an order of magnitude. CTA will provide energy coverage from ~20 GeV to more than 300 TeV. The Schwarzschild-Couder (SC) medium size (9.5m) telescopes will feature a novel aplanatic two-mirror optical design capable of accommodating a wide field-of-view with significantly improved angular resolution as compared to the traditional Davies-Cotton optical design. A full-scale prototype SC medium size telescope structure has been designed and will be constructed at the Fred Lawrence Whipple Observatory in southern Arizona during the fall of 2015. concentrate on the novel features of the design.
We report on the characterization of candidate light sensors for use in the next-generation Imaging Atmospheric Cherenkov Telescope project called Cherenkov Telescope Array, a major astro-particle physics project of about 100 telescopes that is currently in the prototyping phase. Our goal is to develop with the manufacturers the best possible light sensors (highest photon detection efficiency, lowest crosstalk and afterpulsing). The cameras of those telescopes will be based on classical super-bi-alkali Photomultiplier tubes but also Silicon Photomultipliers are candidate light sensors. A full characterisation of selected sensors was done. We are working in close contact with several manufacturers, giving them feedback and suggesting improvements.
The Cherenkov Telescope Array is the next generation ground-based observatory for the study of very-high-energy gamma-rays. It will provide an order of magnitude more sensitivity and greater angular resolution than present systems as well as an increased energy range (20 GeV to 300 TeV). For the high energy portion of this range, a relatively large area has to be covered by the array. For this, the construction of ~7 m diameter Cherenkov telescopes is an option under study. We have proposed an innovative design of a Davies-Cotton mount for such a telescope, within Cherenkov Telescope Array specifications, and evaluated its mechanical and optical performance. The mount is a reticulated-type structure with steel tubes and tensioned wires, designed in three main parts to be assembled on site. In this work we show the structural characteristics of the mount and the optical aberrations at the focal plane for three options of mirror facet size caused by mount deformations due to wind and gravity.
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