ASTROMETRY TECHNIQUES FOR THE CALIBRATION OF THE ASTRI TELESCOPE WITH THE VARIANCE METHOD

In the study of Very High-Energy (VHE) astrophysical phenomena the next generation of Imaging Atmospheric Cherenkov Telescopes (IACT) will play a key role thanks to specific ground-based astronomical observations. In this context, the ASTRI project developed a novel instrument endowed with a Schwarzschild-Couder dual-mirror optical configuration (that has never been adopted before in gamma-ray astronomy) and a dedicated Cherenkov camera entirely designed by the Istituto Nazionale di Astrofisica (INAF) based on SiPM sensors. The prototype telescope ASTRI– Horn is located in Italy and carried out successfully in 2019 the techno- logy validation phase, paving the way for the realization of the MiniAr- ray: 9 identical telescopes working in stereoscopic mode to be installed in Tenerife (Canary Islands) within the next three years. However, several issues related to the pointing performances emerged during operations with ASTRI–Horn. Actually, the pointing calibration is generally a critical aspect for Cherenkov telescopes, as their cameras are designed for the detection of nanosecond atmospheric flashes rather than for imaging the starfield and, consequently, it is impossible to use the standard astrometry of the focal plane. Furthermore, in the case of ASTRI, the compactness of the mechanical structure prevents from installing an auxiliary monitoring camera sharing the same optical system of the telescope. Despite these difficulties, the optimization of the pointing performances is crucial for ensuring the scientific accuracy of the whole system. The present PhD thesis aims at the development and validation of new astrometric techniques for the pointing calibration of the ASTRI telescope exploiting the so-called Variance method, a statistical algorithm implemented in the Cherenkov camera electronic board. Thanks to the Variance, the AS- TRI telescope is endowed with an ancillary output owning the potentiality to image the stellar component of the night sky background in the Field of View (FoV), with a quite coarse angular resolution (~11°, corresponding to the pixel size of the Cherenkov camera), but a relatively good sensitivity for an IACT (visual magnitude limit ~7). As we discuss in this document the Variance constitutes a unique opportunity for enhancing the pointing performances of the telescope, and we demonstrate that our procedures offer a chance to reach the critical accuracy level required for achieving the scientific objectives of the ASTRI project. Unfortunately, in this period the COVID-19 pandemic and other accidental events, heavily delayed the maintenance operations on the ASTRI– Horn telescope, and up to now it is still impossible to make new observations dedicated to the validation of our procedures, hence only data taken in previous months were used. As in any other experimental activity, new data taken on purpose would have considerably facilitated our work, but due to the present situation, we focused our attention on Variance data available in the ASTRI archive that has never been explored before. The resulting work represents the first complete and detailed analysis of the Variance method together with its numerous unexplored applications. Our custom astrometry techniques allowed us to reveal that ASTRI–Horn was affected by two kinds of systematic errors, that we characterized and measured for the first time. The experience gained with archive data allowed us to understand how to apply our routines for calibrating the incoming ASTRI MiniArray, indicating an effective strategy to match the crucial requirement for the pointing accuracy. The resulting procedure has already been inserted into the calibration plan of the MiniArray and its Online Observation Quality System (OOQS). The structure of the present document is articulated in eight chapters and three appendices, whose content can be summarized as follows. Chapter 1 presents the status of the art in VHE astrophysics, focusing on the observational features of cosmic rays and gamma rays, together with a description of the main open questions in this research field. Chapter 2 is dedicated to IACTs, presenting their history and operating principles, and introducing the major examples of instruments currently in activity worldwide. Chapter 3 focuses on the ASTRI project, presenting both the prototype telescope ASTRI–Horn and the incoming observatory of the MiniArray. In particular, it is reported a detailed description of the most relevant sub-systems for this thesis: the camera, the optical scheme, and the pointing strategy. Chapter 4 goes into the details of the Variance method. A technical description of its functioning at the electronic level is provided at first, while the core of the chapter is dedicated to our routines for the production of sky images and their calibration. Chapter 5 reports specific tools and procedures that we developed for the analysis of Variance images: the astrometric calibration, the de-convolution of the star signal, and the transformation function to correct the artifacts introduced by the geometric arrangement of the pixels. Chapter 6 describes the algorithm to assess the alignment of the Cherenkov camera to the optical axis of the telescope exploiting the apparent rotation of the FoV during long observing runs in tracking mode. Chapter 7 shows a custom procedure for the star identification developed on purpose for Variance images (as it is impossible to adopt the standard astrometry software for their analysis) allowing to monitor in real-time the actual pointing direction of the telescope. Chapter 8 contains the concluding remarks. It summarizes the main results achieved in this thesis, highlighting their importance but also some limitations and suggesting further improvements. Future perspectives of this work are briefly presented at last, with particular attention to its implementation on the incoming ASTRI MiniArray. At the end of this document, three appendices report additional/complementary material concerning respectively metrological techniques for the inspection of shape and reflectivity of primary mirror segments (A), more details about the software developed for this thesis and the access to it (B), and the massive activity of outreach and education carried out during the doctoral period in the field of Cherenkov astronomy (C).