First optical validation of a Schwarzschild Couder telescope: the ASTRI SST-2M Cherenkov telescope

¹ INAF Osservatorio Astronomico di Padova, vicolo dell’ Osservatorio 5, 35122 Padova, Italy
e-mail: enrico.giro@oapd.inaf.it
² INAF Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy
³ Dipartimento di Fisica e Geologia, Universitá di Perugia, via Pascoli, 06123 Perugia, Italy
⁴ Instituto de Astronomia, Geofisica e Ciências Atmosfericas, Universidade de São Paulo, 1226 Rua do Matão, São Paulo, Brasil
⁵ INAF IASF Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ INAF Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁸ INAF Osservatorio Astrofisico di Catania, via S. Sofia 78, 95125 Catania, Italy
⁹ Department of Physics / Astronomy, University of California, 430 Portola Plaza, Los Angeles, CA 90095, USA

Received: 19 July 2017
Accepted: 8 September 2017

Abstract

Context. The Cherenkov Telescope Array (CTA) represents the most advanced facility designed for Cherenkov Astronomy. ASTRI SST-2M has been developed as a demonstrator for the Small Size Telescope in the context of the upcoming CTA. Its main innovation consists in the optical layout which implements the Schwarzschild-Couder configuration and is fully validated for the first time. The ASTRI SST-2M optical system represents the first qualified example of a two-mirror telescope for Cherenkov Astronomy. This configuration permits us to (i) maintain high optical quality across a large field of view; (ii) demagnify the plate scale; and (iii) exploit new technological solutions for focal plane sensors.

Aims. The goal of the paper is to present the optical qualification of the ASTRI SST-2M telescope. The qualification has been obtained measuring the point spread function (PSF) sizes generated in the focal plane at various distances from the optical axis. These values have been compared with the performances expected by design.

Methods. After an introduction on Gamma-ray Astronomy from the ground, the optical design of ASTRI SST-2M and how it has been implemented is discussed. Moreover, the description of the set-up used to qualify the telescope over the full field of view is shown.

Results. We report the results of the first–light optical qualification. The required specification of a flat PSF of ~ 10 arcmin in a large field of view (~ 10°) has been demonstrated. These results validate the design specifications, opening a new scenario for Cherenkov Gamma-ray Astronomy and, in particular, for the detection of high-energy (5–300 TeV) gamma rays and wide-field observations with CTA.