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

Abstract

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 for two mirrors telescope for Cherenkov Astronomy. This configuration permits to (i) maintain a high optical quality across a large FoV (ii) de-magnify the plate scale, (iii) exploit new technological solutions for focal plane sensors. The goal of the paper is to present the optical qualification of the ASTRI SST-2M telescope. The qualification has been obtained measuring the PSF sizes generated in the focal plane at various distance from the optical axis. These values have been compared with the performances expected by design. After an introduction on the Gamma Astronomy from the ground, the optical design and how it has been implemented for ASTRI SST-2M is discussed. Moreover the description of the setup used to qualify the telescope over the full field of view is shown. We report the results of the first--light optical qualification. The required specification of a flat PSF of $\sim 10$ arcmin in a large field of view ~10 deg 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.

arXiv comment6 pages, 5 figures

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• Detection of the Crab Nebula with the 9.7 m Prototype Schwarzschild-Couder Telescope
• Mirror production for the Cherenkov telescopes of the ASTRI Mini-Array and of the MST project for the Cherenkov Telescope Array
• Novel Silicon Photomultipliers suitable for Dual-Mirror Small-Sized Telescopes of the Cherenkov Telescope Array
• Detectors for high-energy messengers from the Universe
• Assessment of the Cherenkov camera alignment through Variance images for the ASTRI telescope
• Effective pointing of the ASTRI-Horn telescope using the Cherenkov camera with the Variance method
• Calibration and monitoring of the ASTRI-Horn telescope by using the night-sky background measured by the photon-statistics ("variance") method
• Muon calibration of the ASTRI-Horn telescope: preliminary results
• Advanced modelling of Night Sky Background light for Imaging Atmospheric Cherenkov Telescopes
• Multivariate time-series forecasting of ASTRI-Horn monitoring data: A Normal Behavior Model