[Paper Review] Boosting the performance of the ASTRI SST-2M prototype: reflective and anti-reflective coatings

G. Bonnoli, R. Canestrari+4 more|arXiv (Cornell University)|Jul 20, 2013

Spacecraft Design and Technology3 citations

TL;DR

This paper presents the development and optimization of specialized dielectric coatings for the ASTRI SST-2M prototype Cherenkov telescope, including a low-pass reflective coating for primary/secondary mirrors and an anti-reflective coating for pyramidal light guides. The coatings significantly enhance signal-to-noise performance by filtering night sky background light and improving photon collection, with simulations and measurements confirming a 6.5 quality factor (Q) and up to 90.6% reflectivity in the 300–550 nm band.

ABSTRACT

ASTRI is a Flagship Project of the Italian Ministry of Education, University and Research, led by the Italian National Institute of Astrophysics, INAF. One of the main aims of the ASTRI Project is the design, construction and verification on-field of a dual mirror (2M) end-to-end prototype for the Small Size Telescope (SST) envisaged to become part of the Cherenkov Telescope Array. The ASTRI SST-2M prototype adopts the Schwarzschild-Couder design, and a camera based on SiPM (Silicon Photo Multiplier); it will be assembled at the INAF astronomical site of Serra La Nave on mount Etna (Catania, Italy) within mid 2014, and will start scientific validation phase soon after. The peculiarities of the optical design and of the SiPM bandpass pushed towards specifically optimized choices in terms of reflective coatings for both the primary and the secondary mirror. In particular, multi-layer dielectric coatings, capable of filtering out the large Night Sky Background contamination at wavelengths $λ\gtrsim 700$ nm have been developed and tested, as a solution for the primary mirrors. Due to the conformation of the ASTRI SST-2M camera, a reimaging system based on thin pyramidal light guides could be optionally integrated aiming to increase the fill factor. An anti-reflective coating optimized for a wide range of incident angles faraway from normality was specifically developed to enhance the UV-optical transparency of these elements. The issues, strategy, simulations and experimental results are thoroughly presented.

Motivation & Objective

• Address the challenge of high night sky background (NSB) contamination in the 700–900 nm range, which degrades Cherenkov signal detection due to SiPMs' extended sensitivity.
• Develop multi-layer dielectric coatings for primary and secondary mirrors that act as low-pass filters to suppress long-wavelength NSB while maintaining high reflectivity in the 300–550 nm band.
• Design and test an anti-reflective coating for LAK9 glass light guides in a reimaging system to minimize losses from total internal reflection at oblique angles (20°–70° incidence).
• Optimize the entire optical chain by integrating mirror coatings, light guide coatings, and SiPM response to maximize Cherenkov signal acceptance and NSB rejection.
• Evaluate coating performance using a quality factor Q = A/(1−B), where A is signal acceptance and B is NSB rejection, to quantify system-level improvements over baseline Al+SiO₂ coatings.

Proposed method

• Design multi-layer dielectric coatings using physical vapor deposition (PVD) of materials like SiO₂, TiO₂, Ta₂O₅, and ZrO₂ to create interference-based filters with high reflectivity in the 300–550 nm band and strong suppression beyond 700 nm.
• Simulate angular-dependent reflectivity for primary and secondary mirrors at incidence angles of 20°, 40°, and 60° using interference theory and transfer matrix methods.
• Develop a SiO₂/ZrO₂ multilayer anti-reflective coating for LAK9 glass light guides to minimize reflection losses at oblique angles (20°–70°) and enhance UV-optical transmission.
• Fabricate and test coating samples at ZAOT S.r.l., measuring transmission under normal incidence and comparing with simulations to validate performance.
• Integrate coating performance data with SiPM photon detection efficiency (PDE) to compute system-level metrics: signal acceptance (A), NSB rejection (B), and quality factor Q = A/(1−B).
• Use angular-resolved simulations to ensure coating performance matches the optical path across the wide 4.8° field of view, especially for off-normal incidence on the focal surface.

Experimental results

Research questions

• RQ1 How can dielectric coatings on the primary and secondary mirrors of the ASTRI SST-2M prototype effectively suppress night sky background (NSB) light above 700 nm while maintaining high reflectivity in the 300–550 nm band?
• RQ2 What is the optimal multi-layer dielectric coating design for LAK9 glass light guides to minimize reflection losses at oblique angles (20°–70°) in the reimaging system?
• RQ3 How do the angular-dependent optical properties of the coatings affect the overall signal-to-noise performance of the ASTRI SST-2M prototype?
• RQ4 To what extent do the proposed coatings improve the system-level quality factor Q = A/(1−B) compared to the baseline Al+SiO₂ coating?
• RQ5 Can the combination of mirror coatings and anti-reflective light guide coatings significantly enhance the effective fill factor and photon collection efficiency of the SiPM-based camera?

Key findings

• The SiO₂ + TiO₂ multi-layer coating for the primary mirror achieves a quality factor Q of 5.9, representing a 44% improvement over the baseline Al+SiO₂ coating (Q = 4.1).
• The SiO₂ + ZrO₂ anti-reflective coating for light guides reduces reflection losses across a wide angular range (20°–70°), with simulations showing improved transmission compared to uncoated glass.
• Measured transmission of coated LAK9 glass samples under normal incidence matches simulation predictions within expected tolerances, validating the coating design for real-world deployment.
• The proposed SiO₂ + mixture multi-layer coating achieves 90.6% reflectivity in the 300–550 nm band, outperforming the SiO₂ + TiO₂ coating (79.7%) and approaching the baseline Al+SiO₂ coating (91.9%).
• The system-level analysis shows that the optimized coating design increases Cherenkov signal acceptance (A) to 21.9% and NSB rejection (B) to 96.7%, resulting in a Q value of 6.5, the highest among all tested configurations.
• The integration of angular-dependent coating performance with SiPM PDE modeling confirms that the optical chain enhancements significantly reduce background contamination while preserving signal throughput.