Boosting the performance of the ASTRI SST-2M prototype: reflective and anti-reflective coatings

摘要

ASTRI is a Flagship Project of the Italian Ministry of Education, Universityand Research, led by the Italian National Institute of Astrophysics, INAF. Oneof the main aims of the ASTRI Project is the design, construction andverification on-field of a dual mirror (2M) end-to-end prototype for the SmallSize Telescope (SST) envisaged to become part of the Cherenkov Telescope Array.The ASTRI SST-2M prototype adopts the Schwarzschild-Couder design, and a camerabased on SiPM (Silicon Photo Multiplier); it will be assembled at the INAFastronomical site of Serra La Nave on mount Etna (Catania, Italy) within mid2014, and will start scientific validation phase soon after. The peculiaritiesof the optical design and of the SiPM bandpass pushed towards specificallyoptimized choices in terms of reflective coatings for both the primary and thesecondary mirror. In particular, multi-layer dielectric coatings, capable offiltering out the large Night Sky Background contamination at wavelengths$\lambda \gtrsim 700$ nm have been developed and tested, as a solution for theprimary mirrors. Due to the conformation of the ASTRI SST-2M camera, areimaging system based on thin pyramidal light guides could be optionallyintegrated aiming to increase the fill factor. An anti-reflective coatingoptimized for a wide range of incident angles faraway from normality wasspecifically developed to enhance the UV-optical transparency of theseelements. The issues, strategy, simulations and experimental results arethoroughly presented.