Optical design study, testing and qualification of a Schwarzschild-Couder telescope for CTA and an assessment on the Intensity Interferometry capabilities with CTA

There is a growing common effort in the very high energy community towards the development of new research infrastructures to answer the fundamental questions of modern high-energy astrophysics and astroparticle physics. The Cherenkov Telescope Array Observatory (CTAO) is an international project aiming to deploy two separate arrays to observe the whole VHE sky between E = 20 GeV up to E = 300 TeV in a long term plan of about 30 years of operations. CTA is designed to increase the sensitivity by a factor 10 at 1 TeV, to enlarge the detection area, the angular resolution and the field of view over the facilities operating today. The observatory will be characterized by high flexibility, enhanced monitoring and deep survey capabilities, short time scale and simultaneous observations in multiple fields. This PhD thesis addresses the optical design study and testing of dual mirror Imaging Atmospheric Cherenkov Telescopes (IACTs) for the incoming CTAO. All of the IACTs facilities currently operating rely on single mirror solutions, which are mostly parabolic or Davies-Cotton optical designs, however there is a novel interest in the development of dual mirror configurations following the Schwarzschild-Couder optical design. This peculiar design, based on two highly aspherical mirrors promises wide-field, aplanatic telescopes characterized by small f-numbers and more compact structures. Dual mirror solutions allow use smaller camera pixels (3-6 mm) based on Silicon Photo Multiplier technology in substitution of the larger Photo Multiplier Tubes (1 inch) currently in use. The increased complexity in terms of optics manufacturing, replication and alignment is motivated from the attractive new capabilities of such configuration. In this context the Italian National Institute for Astrophysics (INAF) supported by the Italian Ministry of Education, University and Research (MIUR), is developing a small sized telescope prototype for CTA, named ASTRI, which is based upon the Schwarzschild-Couder optical design. The present work deals with the challenging realization of this optical configuration that has never been applied to IACTs. After two introductory chapters on the gamma-ray astronomy and the ASTRI optical design and its main subsystems (chapters 1 and 2), the performances of this system are compared with those of the other common wide-field telescopes in use for Cherenkov observations and for other applications in astrophysics (chapter 3). This comparative study is based on a commercial ray tracing software into which the optical designs of the envisaged telescopes are reproduced. Subsequently in chapter 4, an extended study of the ASTRI capabilities in relation to the performance and environmental requirements issued by CTA is presented in a detailed analysis of compliance supported by ray tracing simulations, finite element analysis and tolerance studies. In chapter 5 the work on the qualification tests of the secondary mirror gives an insight into the complexity of the Schwarzschild-Couder optics. The realization of this optical element is challenging in relation to currently available technologies, in particular concerning the cost requirements imposed by the CTA project. These constraints and the large sagitta of the mirror (190 mm) requires the use of the hot slumping technique in substitution of the cold slumping and diamond milling approaches usually used in the manufacturing of mirrors for Cherenkov applications. The results of a careful and extended test campaign on a mirror prototype have indicated that this manufacturing technique can provide a reliable engineering process of production for such large, highly aspherical optics. With a perspective on the science with future large telescopes as those provided by CTA, an assessment study upon the potentialities of the Intensity Interferometry (II) technique is carried out in chapter 6. In particular, a new kind of observation based on II is explored; the method aims to estimate the direct distance of the celestial objects. The order of magnitudes of the problem parameters space and the sensitivity that CTA and other future large observatories should achieve is estimated by means of numerical simulations. A short-term concept of experiment to assess the reliability of this new method is also discussed in relation to a pilot measurement that could be pursued with the state of the art technology

Models for the active optics system of the ASTRI SST-2M prototype proposed for the Cherenkov Telescope Array

ASTRI SST-2M is an end-to-end prototype of Small Size class of Telescope proposed for the Cherenkov Telescope Array (CTA). Currently under completion at the Serra La Nave observing station (Mt. Etna, Catania, Italy), the ASTRI SST- 2M telescope is the first imaging dual-mirror telescope ever realized for Cherenkov telescopes. A mini-array of nine such telescopes will form the ASTRI mini-array proposed as a precursor and initial seed of CTA to be installed at the final CTA southern site. ASTRI SST-2M is equipped with an active optics system, controlling both the segmented primary mirror and the monolithic secondary mirror, which allows optical re-alignment during telescope slew. We describe the hardware and software solution that have been implemented for optics control and the models we developed for the system

Spectropolarimetry of Galactic stars with anomalous extinction sightlines

Highly reddened type Ia Supernovae (SNe Ia) with low total-to-selective visual extinction ratio values, $R_V$, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths ($\lambda_{max} \lesssim 0.4 \mu m$). It is not clear why sightlines to SNe Ia display such different continuum polarization profiles from interstellar sightlines in the Milky Way with similar $R_V$ values. We investigate polarization profiles of a sample of Galactic stars with low $R_V$ values, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in SN Ia sightlines. We undertook spectropolarimetry of 14 stars, and used archival data for three additional stars, and run dust extinction and polarization simulations to infer a simple dust model that can reproduce the observed extinction and polarization curves. Our sample of Galactic stars with low $R_V$ values and anomalous extinction sightlines displays normal polarization profiles with an average $\lambda_{max} \sim 0.53 {\mu m}$, and is consistent within 3$\sigma$ to a larger coherent sample of Galactic stars from literature. Despite the low $R_V$ values of dust towards the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed towards SNe Ia with low $R_V$ values. There is a correlation between the best-fit Serkowski parameters $K$ and $\lambda_{max}$, but we did not find any significant correlation between $R_V$ and $\lambda_{max}$. Our simulations show that the $K-\lambda_{max}$ relationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normal $\lambda_{max}$ and low $R_V$ values, a population of large (a $\geq 0.1 \mu m$) interstellar silicate grains must be contained in the dust's composition.Comment: accepted for publication in A&

MORFEO enters final design phase

MORFEO (Multi-conjugate adaptive Optics Relay For ELT Observations, formerly MAORY), the MCAO system for the ELT, will provide diffraction-limited optical quality to the large field camera MICADO. MORFEO has officially passed the Preliminary Design Review and it is entering the final design phase. We present the current status of the project, with a focus on the adaptive optics system aspects and expected milestones during the next project phase

Optical design study, testing and qualification of a Schwarzschild-Couder telescope for CTA and an assessment on the Intensity Interferometry capabilities with CTA

There is a growing common effort in the very high energy community towards the development of new research infrastructures to answer the fundamental questions of modern high-energy astrophysics and astroparticle physics. The Cherenkov Telescope Array Observatory (CTAO) is an international project aiming to deploy two separate arrays to observe the whole VHE sky between E = 20 GeV up to E = 300 TeV in a long term plan of about 30 years of operations. CTA is designed to increase the sensitivity by a factor 10 at 1 TeV, to enlarge the detection area, the angular resolution and the field of view over the facilities operating today. The observatory will be characterized by high flexibility, enhanced monitoring and deep survey capabilities, short time scale and simultaneous observations in multiple fields. This PhD thesis addresses the optical design study and testing of dual mirror Imaging Atmospheric Cherenkov Telescopes (IACTs) for the incoming CTAO. All of the IACTs facilities currently operating rely on single mirror solutions, which are mostly parabolic or Davies-Cotton optical designs, however there is a novel interest in the development of dual mirror configurations following the Schwarzschild-Couder optical design. This peculiar design, based on two highly aspherical mirrors promises wide-field, aplanatic telescopes characterized by small f-numbers and more compact structures. Dual mirror solutions allow use smaller camera pixels (3-6 mm) based on Silicon Photo Multiplier technology in substitution of the larger Photo Multiplier Tubes (1 inch) currently in use. The increased complexity in terms of optics manufacturing, replication and alignment is motivated from the attractive new capabilities of such configuration. In this context the Italian National Institute for Astrophysics (INAF) supported by the Italian Ministry of Education, University and Research (MIUR), is developing a small sized telescope prototype for CTA, named ASTRI, which is based upon the Schwarzschild-Couder optical design. The present work deals with the challenging realization of this optical configuration that has never been applied to IACTs. After two introductory chapters on the gamma-ray astronomy and the ASTRI optical design and its main subsystems (chapters 1 and 2), the performances of this system are compared with those of the other common wide-field telescopes in use for Cherenkov observations and for other applications in astrophysics (chapter 3). This comparative study is based on a commercial ray tracing software into which the optical designs of the envisaged telescopes are reproduced. Subsequently in chapter 4, an extended study of the ASTRI capabilities in relation to the performance and environmental requirements issued by CTA is presented in a detailed analysis of compliance supported by ray tracing simulations, finite element analysis and tolerance studies. In chapter 5 the work on the qualification tests of the secondary mirror gives an insight into the complexity of the Schwarzschild-Couder optics. The realization of this optical element is challenging in relation to currently available technologies, in particular concerning the cost requirements imposed by the CTA project. These constraints and the large sagitta of the mirror (190 mm) requires the use of the hot slumping technique in substitution of the cold slumping and diamond milling approaches usually used in the manufacturing of mirrors for Cherenkov applications. The results of a careful and extended test campaign on a mirror prototype have indicated that this manufacturing technique can provide a reliable engineering process of production for such large, highly aspherical optics. With a perspective on the science with future large telescopes as those provided by CTA, an assessment study upon the potentialities of the Intensity Interferometry (II) technique is carried out in chapter 6. In particular, a new kind of observation based on II is explored; the method aims to estimate the direct distance of the celestial objects. The order of magnitudes of the problem parameters space and the sensitivity that CTA and other future large observatories should achieve is estimated by means of numerical simulations. A short-term concept of experiment to assess the reliability of this new method is also discussed in relation to a pilot measurement that could be pursued with the state of the art technology.Vi é un crescente interesse nella comunita’ dell'astrofisica delle alte energie verso lo sviluppo di nuove infrastrutture di ricerca per rispondere alle questioni fondamentali della moderna astrofisica delle alte energie e della fisica delle astroparticelle. Il Cherenkov Telescope Array é un progetto internazionale che ha lo scopo di costruire due array separati per osservare l'intero cielo alle altissime energie tra 20 GeV e 300 TeV lungo un periodo di attivita’ di almeno 30 anni. CTA é pensato per aumentare la sensibilita’ di un fattore 10 ad 1 TeV, per allargare l'area di detection, la risoluzione angolare e il campo di vista rispetto ai telescopi attualmente operanti. L'osservatorio sara’ caratterizzato da un'elevata flessibilita’, potenziate capacita’ di monitoraggio e survey profonde, osservazioni a piccole scale temporali e osservazioni simultanee in campi multipli. Questa tesi dottorato si concentra sullo studio del disegno ottico e dei test di un telescopio Cherenkov a due specchi per l'osservatorio CTA. Sebbene tutti i telescopi Cherenkov operanti oggigiorno, si basino su uno specchio singolo, perlopiu’ secondo disegni parabolici o Davies-Cotton, c’é un nuovo interesse nello sviluppo di configurazioni a due specchi secondo la configurazione Schwarzschild-Couder. Questo disegno peculiare, basato su due specchi altamente asferici definisce dei telescopi aplanatici con ampi campi di vista caratterizzati da f/# molto piccoli (sistemi molto veloci) e strutture piu’ compatte. Soluzioni a due specchi consentono l'uso di pixels piu’ piccoli (3-6 mm) basati sulla tecnologia dei foto-moltiplicatori al silicio in sostituzione ai piu’ grandi tubi foto-moltiplicatori (1 pollice) attualmente in uso. L'aumento di complessita’ in termini di lavorazione delle ottiche, replica ed allineamento é motivato dalle promettenti, nuove, performance di questa configurazione. In questo contesto l'Istituto Nazionale per l'Astrofisica (INAF) supportato dal Ministero Italiano per l'Educazione, l'Universita’ e la Ricerca (MIUR), sta sviluppando un telescopio di piccola taglia per CTA, chiamato ASTRI, e basato su un disegno ottico Schwarzschild-Couder. Il presente lavoro si confronta con la difficile realizzazione di questa configurazione ottica che non é mai stata applicata ai telescopi Cherenkov. Dopo un capitolo introduttivo sull'astronomia gamma e sul disegno ottico di ASTRI (capitoli 1 e 2), le performance di questo sistema ottico sono comparate con quelle dei comuni telescopi a grande campo in uso per osservazioni Cherenkov e per altre applicazioni in astrofisica (capitolo 3). Successivamente nel capitolo 4, un esteso studio delle caratteristiche e delle performance di ASTRI in relazione ai requisiti di performance e ambientali richiesti da CTA é presentato in una dettagliata analisi di compatibilita’ supportata da simulazioni di ray tracing, analisi agli elementi finiti e studi di tolleranza. Nel capitolo 5 i test di qualifica dello specchio secondario danno una visione della complessita’ delle ottiche Schwarzschild-Couder. La realizzazione di questo elemento ottico é difficile in relazione alle tecnologie esistenti e in particolare agli stretti vincoli di costo imposti dal progetto CTA. Questi vincoli e la grande sagitta dello specchio (190 mm) richiedono l'uso della tecnica dello slumping a caldo in sostituzione dello slumping a freddo o della fresatura a diamante generalmente usati per la produzione di ottiche Cherenkov. I risultati di un'estesa campagna di test su alcuni prototipi di specchio ha accertato che questa tecnica di produzione puo’ supportare un affidabile processo di ingegnerizzazione per grandi specchi altamente asferici. Con uno sguardo alla scienza con i futuri grandi telescopi come CTA, é stato effettuato anche uno studio di valutazione sulle potenzialita’ della tecnica dell'interferometria di intensita’ (riportato nel capitolo 6). In particolare, é stato investigato un nuovo tipo di osservazione basato sull'interferometria di intensita’; questo metodo si propone di stimare la distanza geometrica degli oggetti celesti. Gli ordini di grandezza dello spazio dei parametri del problema e la sensibilita’ che CTA e altre grandi osservatori del futuro dovrebbero raggiungere sono stimati a mezzo di simulazioni numeriche. In questa cornice é anche discusso un possibile esperimento per testare la precisione del metodo con le attuali tecnologie a disposizione

Atmospheric modeling by SCRAT experiment during the 2010 ESA BEXUS 10 Flight Campaign

This paper discusses the atmospheric analyses that were carried out using pressure and temperature data collected during the flight of the SCRAT experiment onboard the BEXUS 10 balloon. Here are underlined the vast perspectives in terms of scientific data evaluation, analysis and modeling that can be retrieved by the use of simple commercial temperature and pressure sensors. An environmental model accounting for radiative and convective energy transfer based on the temperature and pressure data has been developed. The diurnal effects on the stratospheric temperature and other related physical quantities as the lapse rate and the water vapor content have been derived. As a by product, the onboard temperature sensor used as a Sun sensor can infer many information on the attitude of the balloon by the study of the temperature oscillations induced by the balloon\u2019s rotation during the floating phase