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Modeling a Dual Mirror Cherenkov Telescope to Analyse Pointing Precision

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Modeling a Dual Mirror Cherenkov Telescope to Analyse Pointing Precision VU University Amsterdam MSc Physics Particle and Astroparticle Physics Master Thesis Modeling a dual mirror Cherenkov telescope to analyse pointing precision by Gijsbert Tijsseling 1853562 August 2014 Supervisor: Daily Supervisor: Examiner: Dr. David Berge Arnim Balzer Dr. Jacco Vink Abstract Modeling a dual mirror Cherenkov telescope to analyse pointing precision by Gijsbert Tijsseling The Cherenkov Telescope Array (CTA) is a fourth generation Imaging Atmospheric Cherenkov Technique (IACT) telescope array currently in its prototyping phase. As demonstrated by current experiments, such as H.E.S.S. or MAGIC, the IACT has enor- mous potential. CTA will succeed its predecessors in every aspect, e.g. energy range, sensitivity, field of view or angular resolution. The array will include telescopes of three different sizes. The Small Sized Telescope (SST) will assure an unrivalled coverage of the high energy part of the electromagnetic spectrum (from 1 Tev to above 100 TeV). The GATE CHEC Telescope (GCT) will use a dual mirror design to ensure low costs and a large field of view (∼9◦) of the Cherenkov camera. The dual mirror design requires a new pointing method for these type of telescopes which is currently under development. Pointing refers to targeting a location in the sky as precise as possible. By using the Cherenkov camera to measure starlight directly and compare the resulting images to known star charts, a pointing precision below the required 7 arcseconds RMS can be obtained. Extensive MonteCarlo simulations of the point spread function (PSF) and the camera electronics are used to verify that the required precision can be achieved. This includes various PSF models, multiple star fitting and the effect of the Earth's rotation during the fit. Results show that a PSF approximated by a Gaussian is not sufficient to obtain the required precision and a more accurate PSF model is required. Preliminary results of multiple star fitting look promising and show a precision of 12-13 arcseconds RMS. Contents Abstract i Contents ii Introduction 1 1 Gamma Ray Astronomy 3 1.1 Cosmic Rays . 3 1.2 Gamma Rays . 4 1.3 Sources of Gamma Rays . 6 1.3.1 Supernova Remnants . 6 1.3.2 Pulsars . 7 1.3.3 Galactic Centre . 8 1.3.4 Active Galactic Nuclei . 9 1.3.5 Galaxy Clusters . 9 2 Cherenkov Telescopes 11 2.1 Extensive Air Showers . 11 2.1.1 Particle Showers . 11 2.1.2 Cherenkov Emission . 14 2.2 Imaging Atmospheric Cherenkov Technique . 16 2.3 Current Experiments . 18 2.4 Cherenkov Telescope Array . 19 2.5 CTA SST . 20 3 Camera and Electronics 22 3.1 CHEC . 22 3.2 Electronics . 24 3.3 ADC Slow Readout . 26 3.4 Lab Setup . 27 4 Pointing 29 4.1 Current Pointing Method . 30 4.2 Sources of Pointing Deviation . 32 4.2.1 Tracking Deviation . 32 4.2.2 Deformation of the Structure . 32 4.2.3 Inelastic Deformations . 33 4.2.4 Inaccuracies of Image Analysis . 33 4.3 Dual Mirror Pointing . 33 5 MonteCarlo Simulations 35 5.1 Specifications . 35 5.1.1 Telescope and Camera . 35 ii iii 5.1.2 Starlight and NSB . 36 5.2 RooFit . 37 5.3 Point Spread Function . 38 5.3.1 PSF of the H.E.S.S. Telescope . 40 5.3.2 PSF Simulations for the GCT . 41 5.4 Moving a Star over the Camera . 43 5.5 Rotation of the Earth . 45 5.6 Multiple Star Fitting . 46 5.7 Discussion . 49 Summary 51 Acknowledgements 52 Bibliography 53 Introduction Astronomy is one of the first of the natural sciences. It can be traced all the way back to the beginning of civilization. Artefacts have been found from the ancient Chinese, Greeks and Babylonians describing observations of objects visible with the naked eye. In these ancient times astronomy was used for counting days and creating calendars. However, astronomy went through a revolution when the telescope was invented. Thanks to this invention scientist were able to investigate weak sources, visible in the optical, and astronomy could be used for astrometry and celestial navigation. After this big step things moved quickly. New techniques were developed to research other parts of the electromagnetic spectrum and we were able to learn about the Universe. One of the most recent fields of interest is gamma-ray astronomy. This field focuses on the upper end of the electromagnetic spectrum. Gamma-ray astronomy is strongly connected to cosmic rays. Cosmic ray particles arrival directions are isotropic due to their charged nature and interstellar magnetic fields which makes it impossible to identify their source. Gamma rays, however, are not disturbed by deflection and can be traced back in a straight line to their source of origin. Cosmic rays were discovered in 1912 when Victor Hess observed that ionisation rates increased with altitude [1] while the opposite was expected. The measurements were done with balloon experiments carrying electrometers to an height up to 5300 meters. He reported "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above." Hess could even discard the Sun as a possible source by comparing balloon flights at different times during the day and throughout a solar eclipse. In 1936, Hess received the Nobel Price in Physics for his research and this discovery. In 1920, Robbert Millikan referred, for the first time, to this radiation from outer space as cosmic rays. From the first discovery, scientist showed great interest in cosmic rays. Many more experiments followed to investigate their nature and origin. After balloons, also satellite and ground-based experiments were initiated to perform more in-depth research. In parallel, there are initiatives focusing more on cosmic gamma ray detection. In 2004 the H.E.S.S experiment was the first ground-based experiment to spatially resolve a source of cosmic gamma rays [2]. There is still much to discover and improve in gamma-ray astronomy and this is the motivation for CTA [3]. The CTA project is an initiative to build the next generation ground-based very high energy (VHE) gamma-ray instrument. By surpassing all current Cherenkov telescope experiments in sensitivity and precision, 1 2 it will provide a deep insight into the non-thermal high-energy universe and the origin of cosmic rays. To obtain the most accurate physics results, the true direction of the telescope must be known as precise as possible. The systematic uncertainty CTA wishes to ascertain is below 7 arcseconds for the position of a point like sources. This work will investigate a newly proposed pointing method for the SST-2M telescope by performing MonteCarlo simulation. Different models are tested to examine if the required 7 arcseconds can be achieved. Chapter 1 Gamma Ray Astronomy Gamma-ray astronomy is an exceptional field in astronomy. By observing the universe in one of the highest energy ranges (up to PeV) we can detect the most extreme events occurring. At this moment, about 170 gamma ray sources have been detected1. Objects like stars produce radiation through thermal processes and the amount and energy of this radiation is given by temperature. The most energetic radiation produced thermally by extreme hot objects is about 10 keV. Radiation produced with more energy cannot originate from these objects and must have another origin and production process. This significant amount of gamma-ray flux is created by non-thermal processes such as cosmic charged particle acceleration. As the flux is very low for these high energy photons, a specialized detection method is required to observe gamma-ray sources. 1.1 Cosmic Rays After the discovery of the cosmic rays by Hess, many experiments have analysed and investigated this extraterrestrial radiation to determine its origin and composition. The main components are protons, they make up 85% of all cosmic rays. Followed by 12% of alpha particles and 1% of heavier elements and 2% electrons. Remarkable is the matter antimatter ratio. Only 0:01% of the cosmic rays consist of antimatter [4]. This is an indication for the matter-antimatter asymmetry in the Universe. Figure 1.1 shows the energy spectrum of cosmic rays. The spectrum looks remarkably featureless. To enhance the feutures, the spectrum is multiplied with E2:5. The spectrum can mostly be described as a power law dN ∼ E−γ (1.1) dE The γ in Equation 1.1 represents the spectral index and is approximately 2.7. The power spectrum shows two features. One at an energy of 3 − 4 · 1015 eV the spectrum steepens to an index of γ ' 3. This is called the knee [5]. The second feature can be seen at an energy of 3 · 1018 eV and is called the ankle. At 5 · 1019 the spectrum reaches the 1See http://tevcat.uchicago.edu/ for all gamma ray sources 3 Chapter 1. Gamma Ray Astronomy 4 Greisen-Zatsepin-Kuzmin limit (GZK limit) [6]. This is the theoretical upper limit on cosmic ray energy. The limit arises from protons interacting with the cosmic microwave background (CMB). The mean free path is in the order of 50 Mpc. The knee indicates the energy range where the galactic magnetic field cannot longer contain the energetic cosmic rays and they diffuse out of the galaxy. A transition area follows between galactic and extra-galactic cosmic ray particles. From the ankle is the extra-galactic particles will dominate the energy spectrum. Cosmic ray particles with energies above the GZK limit are unable to reach earth unless they originate from sources within ∼50 Mpc. However, particles at these extreme high energies are sparse, resulting in limiting statistics and inconclusive results.
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