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تسجيل مستخدم جديدStrategy Implementation for the CTA Atmospheric Monitoring Program
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The Cherenkov Telescope Array (CTA) is the next generation facility of Imaging Atmospheric Cherenkov Telescopes. It will reach unprecedented sensitivity and energy resolution in very-high-energy gamma-ray astronomy. CTA will detect Cherenkov light emitted within an atmospheric shower of particles initiated by cosmic-gamma rays or cosmic rays entering the Earths atmosphere. From the combination of images the Cherenkov light produces in the telescopes, one is able to infer the primary particle energy and direction. A correct energy estimation can be thus performed only if the local atmosphere is well characterized. The atmosphere not only affects the shower development itself, but also the Cherenkov photon transmission from the emission point in the particle shower, at about 10-20 km above the ground, to the detector. Cherenkov light on the ground is peaked in the UV-blue region, and therefore molecular and aerosol extinction phenomena are important. The goal of CTA is to control systematics in energy reconstruction to better than 10%. For this reason, a careful and continuous monitoring and characterization of the atmosphere is required. In addition, CTA will be operated as an observatory, with data made public along with appropriate analysis tools. High-level data quality can only be ensured if the atmospheric properties are consistently and continuously taken into account. In this contribution, we concentrate on discussing the implementation strategy for the various atmospheric monitoring instruments currently under discussion in CTA. These includes Raman lidars and ceilometers, stellar photometers and others available both from commercial providers and public research centres.
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The Imaging Atmospheric Cherenkov Technique (IACT) is unusual in astronomy as the atmosphere actually forms an intrinsic part of the detector system, with telescopes indirectly detecting very high energy particles by the generation and transport of C
herenkov photons deep within the atmosphere. This means that accurate measurement, characterisation and monitoring of the atmosphere is at the very heart of successfully operating an IACT system. The Cherenkov Telescope Array (CTA) will be the next generation IACT observatory with an ambitious aim to improve the sensitivity of an order of magnitude over current facilities, along with corresponding improvements in angular and energy resolution and extended energy coverage, through an array of Large (23m), Medium (12m) and Small (4m) sized telescopes spread over an area of order ~km$^2$. Whole sky coverage will be achieved by operating at two sites: one in the northern hemisphere and one in the southern hemisphere. This proceedings will cover the characterisation of the candidate sites and the atmospheric calibration strategy. CTA will utilise a suite of instrumentation and analysis techniques for atmospheric modelling and monitoring regarding pointing forecasts, intelligent pointing selection for the observatory operations and for offline data correction.
The Cherenkov Telescope Array (CTA) will be the next generation observatory employing different types of Cherenkov telescopes for the detection of particle showers initiated by very-high-energy gamma rays. A good knowledge of the Earths atmosphere, w
hich acts as a calorimeter in the detection technique, will be crucial for calibration in CTA. Variations of the atmospheres transparency to Cherenkov light and not correctly performed calibration of individual telescopes in the array result in large systematic uncertainties on the energy scale. The Cherenkov Transparency Coefficient (CTC), developed within the H.E.S.S. experiment, quantifies the mean atmosphere transparency ascertained from data taken by Cherenkov telescopes during scientific observations. Provided that atmospheric conditions over the array are uniform, transparency values obtained per telescope can be also used for the calibration of individual telescope responses. The application of the CTC in CTA presents a challenge due to the greater complexity of the observatory and the variety of telescope cameras compared with currently operating experiments, such as H.E.S.S. We present here the first results of a feasibility study for extension of the CTC concept in CTA for purposes of the inter-calibration of the telescopes in the array and monitoring of the atmosphere.
The Cherenkov Telescope Array (CTA) is the next generation of ground-based gamma-ray observatory. The observatory will consist of two arrays, one located in the southern hemisphere (Paranal,Chile) and the other in the northern hemisphere (Canary Isla
nd, Spain), covering the whole sky in the range of observation. More than 100 telescopes are planned to be in operation for as long as 30 years, which motivated the development of a continuous condition monitoring of the individual telescopes. The main goal of the monitoring is to detect degradation and failures before critical damages occur. Two approaches are considered: the structure monitoring system, in which the Eigenfrequencies of the telescope and their damping rates are measured and monitored; and the drive monitoring, in which the power spectra of rotating components are measured during telescope movements. The structure monitoring concept system was applied to the prototype Medium Size telescope (MST) prototype of CTA in Berlin during late 2018 and in 2019, and the first results are presented here. The system showed reasonable stability during periods, in which the telescope structure was unchanged. The system was also capable to detect mechanical changes, e.g. varying tension in the steel ropes of the camera support structure. The successful implementation of the structure monitoring system supports the decision of implementing the system in all future MSTs.
The Cherenkov Telescope Array (CTA) is the next generation ground based observatory for gamma ray astronomy at very high energies. Employing more than 100 Imaging Atmospheric Cherenkov Telescopes in the northern and southern hemispheres, it was desig
ned to reach unprecedented sensitivity and energy resolution. Understanding and correcting for systematic biases on the absolute energy scale and instrument response functions will be a crucial issue for the performance of CTA. The LUPM group and the Spanish/Italian/Slovenian collaboration are currently building two Raman LIDAR prototypes for the online atmospheric calibration along the line of sight of the CTA. Requirements for such a solution include the ability to characterize aerosol extinction at two wavelengths to distances of 30 km with an accuracy better than 5%, within time scales of about a minute, steering capabilities and close interaction with the CTA array control and data acquisition system as well as other auxiliary instruments. Our Raman LIDARs have design features that make them different from those used in atmospheric science and are characterized by large collecting mirrors (2.5 m2), liquid light guides that collect the light at the focal plane and transport it to the readout system, reduced acquisition time and highly precise Raman spectrometers. The Raman LIDARs will participate in a cross calibration and characterization campaign of the atmosphere at the CTA North site at La Palma, together with other site characterization instruments. After a one year test period there, an in depth evaluation of the solutions adopted by the two projects will lead to a final Raman LIDAR design proposal for both CTA sites.
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