Prototyping hexagonal light concentrators using high-reflectance specular films for the large-sized telescopes of the Cherenkov Telescope Array

Abstract

A: 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 Light-Trap: A novel concept for a large SiPM-based pixel for Very High Energy gamma-ray astronomy and beyond

“…The reflectivity of this material to ∼532 nm light was estimated to be of (98 ± 2)%, with no major dependence on the incident angle (García, 2017). Those measurements are compatible with the reflectivity reported in Figure 5 of (Okumura et al, 2017).…”

Development of a blue-mirror multilayer coating on light concentrators for future SiPM cameras

“…Second, the reflectance performance should be optimized for angles of incidence around 65 • and should not be highly dependent on the angles. This is because photons coming from the mirror directions hit the light concentrator surfaces at angles from ∼40 • to 90 • [13]. Third, the number of layers should not exceed ∼10 and only readily available materials, such as SiO 2 and Ta 2 O 5 , should be used.…”

Using Muon Rings for the Calibration of the Cherenkov Telescope Array: A Systematic Review of the Method and Its Potential Accuracy

“…Residual dependencies of several percent per degree remain and need to be taken into account. Moreover, light concentrators in front of the photodetector plate may degrade with time (unless they are kept in the camera housings and are hence not exposed to the outer environment Okumura et al (2017)), particularly the dependency of their acceptance with respect to light incidence angle can change.…”

“…Together with the variation in efficiency of the photodetectors (PMTs or SiPMs) with incidence angle, this can lead to a nontrivial dependence of the reconstructed B µ on ρ R . Photo-detection devices, such as PMTs (Okumura et al 2017) or SiPMs (Otte et al 2017), used for IACT cameras exhibit quite pronounced dependencies of their photon detection efficiencies on incidence angle. IACT camera pixels are, moreover, often preceded by light concentrators (or funnels, often approximating as much as possible the form of a compound parabolic concentrator (CPC); Winston et al (2005) or Bézier curves Okumura (2012)) to reduce dead spaces and enhance overall optical efficiency (Hénault et al 2013;Aguilar et al 2015;Hahn et al 2017;Okumura et al 2017).…”

“…Photo-detection devices, such as PMTs (Okumura et al 2017) or SiPMs (Otte et al 2017), used for IACT cameras exhibit quite pronounced dependencies of their photon detection efficiencies on incidence angle. IACT camera pixels are, moreover, often preceded by light concentrators (or funnels, often approximating as much as possible the form of a compound parabolic concentrator (CPC); Winston et al (2005) or Bézier curves Okumura (2012)) to reduce dead spaces and enhance overall optical efficiency (Hénault et al 2013;Aguilar et al 2015;Hahn et al 2017;Okumura et al 2017). Such light concentrators achieve only approximately uniform photon detection efficiencies up to a carefully chosen maximum incidence angle.…”