Assessment of the GCT prototype's optical system implementation and other key performances for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) project, led by an international collaboration of institutes, aims to create the world's largest next generation observatory for Very High Energy (VHE) gamma-ray astronomy. It will be devoted to observations in a wide band of energy, from a few tens of GeV to a few hundreds of TeV with Large, Medium and Small-sized telescopes. The Small-Size Telescopes (SSTs) are dedicated to the highest energy range above a few TeV and up to 300 TeV. GCT is an imaging atmospheric Cherenkov telescope (IACT) proposed for the subarray of about 70 SSTs to be installed on the Southern site of CTA in Chile. The Observatory of Paris and the National Institute for Earth Sciences and Astronomy (INSU/CNRS) have developed the mechanical structure, mirrors (aspherical lightweight aluminium segments) and control system of the GCT. The GCT is based on a Schwarzschild-Couder (S-C) dual-mirror optical design which has the advantages, compared to the current IACTs, to offer a wide field of view (similar to 9 degrees) while decreasing the cost and volume (similar to 9 m x 4 m x 6 m for similar to 11 tons) of the telescope structure, as well as the camera. The prototype (pGCT) has been installed at the Meudon's site of the Observatory of Paris and was the first S-C telescope and the first CTA prototype to record VHE events on-sky in November 2015. After three years of intensive testing, pGCT has now been commissioned. This paper is a status report on the complete GCT telescope optical system and the performance it can provide for CTA.