[Paper Review] Study on a prototype of the large dimensional refractive lens for the future large field-of-view IACT

Tianqing Chen, Zixuan Wang+16 more|arXiv (Cornell University)|Sep 6, 2015

Optical Systems and Laser Technology3 citations

TL;DR

This paper proposes a large-diameter, thin water-based refractive lens as a novel light collector for next-generation large field-of-view Imaging Atmospheric Cherenkov Telescopes (IACTs). The prototype, a 0.9 m diameter plano-convex lens made of acrylic and purified water, demonstrated focal length and spot size consistent with ZEMAX simulations, achieved >93% transmittance in visible spectrum, and showed potential for a FoV up to 26°×26° with 50% encircled energy, validating its feasibility for high-sensitivity, wide-field gamma-ray detection.

ABSTRACT

In gamma ray astronomy, the energy range from sub-100GeV to TeV is crucial due to where there is a gap between space experiments and ground-based ones. In addition, observations in this energy range are expected to provide more details about the high energy emission from GRBs,and thus to understand EBL. Based on the observation results and the related knowledge, scientists may be able to unveil the mysteries of galaxy formation and the evolution of early universe. One of the principal issues for next generation Imaging Atmospheric Cherenkov Telescopes (IACT) is to achieve larger field of view (FoV). In this work, we report a refractive water convex lens as light collector to test the feasibility of a new generation of IACT, and some preliminary test results on the optical properties (the focal length, spot size, transmittance, etc.) of a 0.9 m diameter water lens, the photodetectors and DAQ system of a prototype are presented and discussed.

Motivation & Objective

• To develop and test a large-diameter refractive lens using purified water as the optical medium for next-generation IACTs.
• To address the scientific need for larger field-of-view and lower energy thresholds in ground-based gamma-ray astronomy, particularly for sub-100 GeV to TeV observations.
• To validate the feasibility of a water lens as a cost-effective, high-transmittance alternative to Fresnel lenses in large-scale IACT systems.
• To demonstrate the optical performance of the prototype, including focal length, spot size, and transmittance, under controlled laboratory conditions.
• To integrate the lens with a photodetector and DAQ system for coincident detection of atmospheric Cherenkov light from very high-energy cosmic rays.

Proposed method

• Design and fabrication of a 0.9 m diameter, plano-convex, thin spherical cap water lens composed of acrylic shell and purified water.
• Measurement of focal length using a distant collimated light source (400 m away) and adjustment of screen distance to locate the sharpest image.
• Determination of spot size via imaging the focused light spot on a screen and measuring its full width at half maximum.
• Transmittance measurements of acrylic and double-layer materials across 310–660 nm using a spectrometer and calibrated light source.
• Use of ZEMAX optical simulation to predict energy collecting efficiency and field-of-view (FoV) performance based on lens transmittance and PMT response.
• Integration of 16 Hamamatsu R7725 PMTs (2-inch, bialkali photocathode) and a 12-bit FADC (DT5742) for signal amplification and digitization in the DAQ system.

Experimental results

Research questions

• RQ1 Can a large, thin water lens achieve sufficient optical performance (focal length, spot size, transmittance) to serve as a viable light collector in a large field-of-view IACT?
• RQ2 What is the transmittance of the acrylic shell and double-layer material in the visible spectrum relevant to atmospheric Cherenkov light?
• RQ3 To what extent can the water lens support a wide field-of-view, and what is the predicted energy collection efficiency across different angles of incidence?
• RQ4 How does the prototype’s photodetector and DAQ system perform in capturing and digitizing Cherenkov light signals from cosmic rays?
• RQ5 What is the expected noise level and signal-to-noise ratio for the system under typical night sky background conditions at high-altitude observatory sites?

Key findings

• The measured focal length of the 0.9 m water lens was consistent with ZEMAX simulation results, confirming accurate optical design and fabrication.
• The spot size of the focused beam was measured to be approximately 1.5 mm FWHM, indicating good spatial resolution for Cherenkov light collection.
• The acrylic shell material exhibited a transmittance of over 93% in the 420–660 nm range, with double-layer material exceeding 86% transmittance.
• The lens achieved a predicted field-of-view of up to 26°×26° for 50% encircled energy, demonstrating potential for large-scale sky coverage.
• The noise counting rate from night sky background was estimated at 150 MHz per PMT, with a detectable signal rate of ~3.5×10⁻² Hz for 100 TeV protons.
• Waveform data from the FADC system confirmed the presence of single photoelectrons and afterpulses (e.g., 18 NPE at 780 ns), indicating functional signal digitization and trigger readiness.