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

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.

First detection of the Crab Nebula at TeV energies with a Cherenkov telescope in a dual-mirror Schwarzschild-Couder configuration: the ASTRI-Horn telescope

“…The telescope equivalent focal length is 2.15 m (f/0.5) and the system covers a full field of view (FoV) of more than 10 • (Rodeghiero et al, 2016). The SC optical design for Cherenkov telescopes has been fully validated for the first time by means of the ASTRI-Horn telescope (Giro et al, 2017). Images of the Polaris Star at different off-axis angles (up to 4.5 • ) have been taken with an optical CCD camera able to scan the whole FoV, demonstrating that the PSF has a constant width of a few arcmin over the whole FoV.…”

The ASTRI-Horn Dual-Mirror Small-Size Cherenkov Telescope: recent updates, first results and outlook

“…The ASTRI-Horn telescope Cornelia Arcaro position along the telescope's field of view and its elevation [11,12], showing that the required specification of a flat PSF of ∼ 10 arcminutes along a large field of view is fulfilled. The monitoring of the mirror reflectivity is crucial for evaluating the degradation of their reflective coating due to the environmental impact.…”

The ASTRI Dual-Mirror Small-Size Cherenkov Telescope

“…The validation of the optical design has been conducted during a dedicated campaign in Autumn 2016. The optics' PSF has been determined as a function of the position along the telescope's field of view [14]. In addition, the temporal stability of the PSF was determined in function of the telescope elevation [9].…”