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Using muon rings for the optical throughput calibration of the SST-1M prototype for the Cherenkov Telescope Array  [PDF]
S. Toscano,E. Prandini,W. Bilnik,J. B?ocki,L. Bogacz,T. Bulik,F. Cadoux,A. Christov,M. Cury?o,D. della Volpe,M. Dyrda,Y. Favre,A. Frankowski,?. Grudnik,M. Grudzińska,M. Heller,B. Id?kowski,M. Jamrozy,M. Janiak,J. Kasperek,K. Lalik,E. Lyard,E. Mach,D. Mandat,A. Marsza?ek,J. Micha?owski,R. Moderski,T. Montaruli,A. Neronov,J. Niemiec,M. Ostrowski,P. Pa?ko,M. Pech,A. Porcelli,M. Rameez,P. Rajda,E. jr. Schioppa,P. Schovanek,K. Seweryn,K. Skowron,V. Sliusar,M. Sowiński,?. Stawarz,M. Stodulska,M. Stodulski,I. Troyano Pujadas,R. Walter,M. Wi?cek,A. Zagdański,K. Zi?tara,P. ?ychowski,for the CTA Consortium
Physics , 2015,
Abstract: Imaging Atmospheric Cherenkov Telescopes (IACTs) are ground-based instruments devoted to the study of very high energy gamma-rays coming from space. The detection technique consists of observing images created by the Cherenkov light emitted when gamma rays, or more generally cosmic rays, propagate through the atmosphere. While in the case of protons or gamma-rays the images present a filled and more or less elongated shape, energetic muons penetrating the atmosphere are visualised as characteristic circular rings or arcs. A relatively simple analysis of the ring images allows the reconstruction of all the relevant parameters of the detected muons, such as the energy, the impact parameter, and the incoming direction, with the final aim to use them to calibrate the total optical throughput of the given IACT telescope. We present the results of preliminary studies on the use of images created by muons as optical throughput calibrators of the single mirror small size telescope prototype SST-1M proposed for the Cherenkov Telescope Array.
Calibration of the Cherenkov Telescope Array  [PDF]
Markus Gaug,Michael Daniel,David Berge,Raquel de los Reyes,Michele Doro,Andreas Foerster,Maria Concetta Maccarone,Dan Parsons,Christopher van Eldik,for the CTA Consortium
Physics , 2015,
Abstract: The construction of the Cherenkov Telescope Array is expected to start soon. We will present the baseline methods and their extensions currently foreseen to calibrate the observatory. These are bound to achieve the strong requirements on allowed systematic uncertainties for the reconstructed gamma-ray energy and flux scales, as well as on the pointing resolution, and on the overall duty cycle of the observatory. Onsite calibration activities are designed to include a robust and efficient calibration of the telescope cameras, and various methods and instruments to achieve calibration of the overall optical throughput of each telescope, leading to both inter-telescope calibration and an absolute calibration of the entire observatory. One important aspect of the onsite calibration is a correct understanding of the atmosphere above the telescopes, which constitutes the calorimeter of this detection technique. It is planned to be constantly monitored with state-of-the-art instruments to obtain a full molecular and aerosol profile up to the stratosphere. In order to guarantee the best use of the observation time, in terms of usable data, an intelligent scheduling system is required, which gives preference to those sources and observation programs that can cope with the given atmospheric conditions, especially if the sky is partially covered by clouds, or slightly contaminated by dust. Ceilometers in combination with all-sky-cameras are plannned to provide the observatory with a fast, online and full-sky knowledge of the expected conditions for each pointing direction. For a precise characterization of the adopted observing direction, wide-field optical telescopes and Raman Lidars are planned to provide information about the height-resolved and wavelength-dependent atmospheric extinction, throughout the field-of-view of the cameras.
Divergent pointing with the Cherenkov Telescope Array for surveys and beyond  [PDF]
Lucie Gérard for the CTA Consortium
Physics , 2015,
Abstract: The galactic and extragalactic surveys are two of the main proposed legacy projects of the Cherenkov Telescope Array (CTA), providing an unbiased view of the Universe at energies above tens of GeV. Considering Cherenkov telescopes' limited field of view ($<10^\circ$), the time needed for those projects is large. The many telescopes of CTA will allow taking full advantage of new pointing modes in which telescopes point slightly offset from one another. This divergent pointing mode leads to an increase of the array field of view ($\sim 14^\circ$ or larger) with competitive performance compared to normal pointing. We present here a study of the performance of the divergent pointing for different array configurations and number of telescopes. We briefly discuss the prospect of using divergent pointing for surveys.
Prototype of the SST-1M Telescope Structure for the Cherenkov Telescope Array  [PDF]
J. Niemiec,W. Bilnik,J. B?ocki,L. Bogacz,J. Borkowski,T . Bulik,F. Cadoux,A. Christov,M. Cury?o,D. della Volpe,M. Dyrda,Y. Favre,A. Frankowski,?. Grudnik,M. Grudzińska,M. Heller,B. Id?kowski,M. Jamrozy,M. Janiak,J. Kasperek,K. Lalik,E. Lyard,E. Mach,D. Mandat,A. Marsza?ek,J. Micha?owski,R. Moderski,T. Montaruli,A. Neronov,M. Ostrowski,P. Pa?ko,M. Pech,A. Porcelli,E. Prandini,P. Rajda,M. Rameez,E. jr. Schioppa,P. Schovanek,K. Seweryn,K. Skowron,V. Sliusar,M. Sowiński,?. Stawarz,M. Stodulska,M. Stodulski,I. Troyano Pujadas,S. Toscano,R. Walter,M. Wi?cek,A. Zagdański,K. Zi?tara,P. ?ychowski for the SST-1M sub-consortium,the CTA Consortium
Physics , 2015,
Abstract: A single-mirror small-size (SST-1M) Davies-Cotton telescope with a dish diameter of 4 m has been built by a consortium of Polish and Swiss institutions as a prototype for one of the proposed small-size telescopes for the southern observatory of the Cherenkov Telescope Array (CTA). The design represents a very simple, reliable, and cheap solution. The mechanical structure prototype with its drive system is now being tested at the Institute of Nuclear Physics PAS in Krakow. Here we present the design of the prototype and results of the performance tests of the structure and the drive and control system.
Camera calibration strategy of the SST-1M prototype of the Cherenokov Telescope Array  [PDF]
E. Prandini,M. Heller,E. Lyard,E. jr. Schioppa,A. Neronov,W. Bilnik,J. B?ocki,L. Bogacz,T. Bulik,F. Cadoux,A. Christov,M. Cury?o,D. della Volpe,M. Dyrda,Y. Favre,A. Frankowski,?. Grudnik,M. Grudzińska,B. Id?kowski,M. Jamrozy,M. Janiak,J. Kasperek,K. Lalik,E. Mach,D. Mandat,A. Marsza?ek,J. Micha?owski,R. Moderski,T. Montaruli,J. Niemiec,M. Ostrowski,P. Pa?ko,M. Pech,A. Porcelli,M. Rameez,P. Rajda,P. Schovanek,K. Seweryn,K. Skowron,V. Sliusar,M. Sowiński,?. Stawarz,M. Stodulska,M. Stodulski,S. Toscano,I. Troyano Pujadas,R. Walter,M. Wi?cek,A. Zagdański,K. Zi?tara,P. ?ychowski,for the CTA Consortium
Physics , 2015,
Abstract: The SST-1M telescope is one of the prototypes under construction proposed to be part of the future Cherenkov Telescope Array. It uses a standard Davis-Cotton design for the optics and telescope structure, with a dish diameter of 4 meters and a large field-of-view of 9 degrees. The innovative camera design is composed of a photo-detection plane with 1296 pixels including entrance window, light concentrators, Silicon Photomultipliers (SiPMs), and pre-amplifier stages together with a fully digital readout and trigger electronics, DigiCam. In this contribution we give a general description of the analysis chain designed for the SST-1M prototype. In particular we focus on the calibration strategy used to convert the SiPM signals registered by DigiCam to the quantities needed for Cherenkov image analysis. The calibration is based on an online feedback system to stabilize the gain of the SiPMs, as well as dedicated events (dark count, pedestal, and light flasher events) to be taken during the normal operation of the prototype.
Studies towards an understanding of global array pointing for the Cherenkov Telescope Array  [PDF]
Stefan Eschbach,Alexander Ziegler,Christopher van Eldik,Johannes Veh,David Berge,Markus Gaug
Physics , 2015,
Abstract: For the proposed Cherenkov Telescope Array (CTA), a post-calibration point-source location accuracy of 3 seconds of arc is aimed for under favorable observing conditions and for gamma-ray energies exceeding 100 GeV. In this contribution, results of first studies on the location accuracy are presented. These studies are based on a toy Monte Carlo simulation of a typical CTA-South array layout, taking into account the expected trigger rates of the different CTA telescope types and the gamma-ray spectrum of the simulated source. With this simulation code it is possible to study the location accuracy as a function of arbitrary telescope mis-orientations and for typical observing patterns on the sky. Results are presented for various scenarios, including one for which all individual telescopes are randomly mis-oriented within their specified limits. The study provides solid lower limits for the expected source location accuracy of CTA, and can be easily extended to include various other important effects like atmospheric refraction or partial cloud coverage.
The Camera Calibration Strategy of the Cherenkov Telescope Array  [PDF]
M. K. Daniel,M. Gaug,P. Majumdar,for the CTA Consortium
Physics , 2015,
Abstract: The Cherenkov Telescope Array (CTA) will be the next generation ground based observatory in very high energy gamma ray astronomy. The facility will achieve a wide energy coverage, starting from a threshold of a few tens of GeV up to hundreds of TeV by utilising several classes of telescopes, each optimised for different regions of the gamma-ray spectrum. The required energy resolution of better than 10-15% over most of the energy range and a goal of 5% systematic uncertainty on the measurement of the Cherenkov light intensity at the position of each telescope means that a very precise evaluation of the entire system will need to be made. The composite nature of the array means multiple camera technologies will be employed so multiple calibration systems and procedures will be necessary to meet the performance requirements. Additional constraints will come from the need to minimise observing time losses and that the observatory is foreseen to operate for tens of years, so both short and long term systematic changes in performance will need to be investigated and monitored. This contribution summarises the recommended camera calibration strategy of CTA based on the experience with current IACTs.
Design of a prototype device to calibrate the Large Size Telescope camera of the Cherenkov Telescope Array  [PDF]
M. Iori,P. Majumdar,F. De Persio,A. Chatterjee,F. Ferrarotto,B. K. Nagesh,L. Saha,B. B. Singh
Physics , 2015,
Abstract: The Cherenkov Telescope Array is a project that aims to exploring the highest energy region of electromagnetic spectrum. Two arrays, one for each hemisphere, will cover the full sky in a range from few tens of GeV to hundreds of TeV improving the sensitivity and angular resolution of the present operating arrays. A prototype of the Large Size Telescope (LST) for the study of gamma ray astronomy above some tens of GeV will be installed at the Canary Island of La Palma in 2016. The LST camera, made by an array of photomultipliers (PMTs), requires an accurate and systematic calibration over a wide dynamic range. In this contribution, we present an optical calibration system made by a 355 nm wavelength laser with 400 ps pulse width, 1 muJ output energy, up to 4k Hz repetition rate and a set of neutral density filters to obtain a wide range of photon intensities, up to 1000 photoelectrons/PMT, to be sent to the camera plane 28 m away. The number of photons after the diffuser of the calibration box, located in the center of the reflective plane, is monitored by a photodiode. The stability of the laser and the ambient parameters inside this calibration box are checked by a multi-task processor and a trigger signal is sent to the camera data acquisition system. The box frame is designed with special attention to obtain a robust device with stable optical and mechanical features.
Real-time atmospheric monitoring for the Cherenkov Telescope Array using a wide-field optical telescope  [PDF]
Jan Ebr,Petr Janecek,Michael Prouza,Jiri Blazek,for the CTA Consortium
Physics , 2015,
Abstract: The Cherenkov Telescope Array (CTA) is the next generation of ground-based very high energy gamma-ray instruments and is planned to be built on two sites (one in each hemisphere) in the coming years, with full array operation foreseen to begin 2020. The goal of performing high precision gamma-ray energy measurements while maximizing the use of observation time demands detailed and fast information about atmospheric conditions. Besides LIDARs designed to monitor clouds and aerosol content of the atmosphere in the pointing direction of the CTA telescopes, we propose to use the "FRAM" (F(/Ph)otometric Robotic Atmospheric Monitor) device, which is a small robotic astronomical telescope with a large field of view and a sensitive CCD camera that together ensure precise atmospheric characterization over the complete field-of-view of the CTA. FRAM will use stellar photometry to measure atmospheric extinction across the field of view of the CTA without interfering with the observation (unlike laser-based methods). This allows FRAM to operate with high temporal resolution and provide both real-time data for on-the-fly scheduling decisions and an offline database for calibration and selection of scientific data. The fast robotic mount of the telescope supports quick observation of multiple fields when the array is split and even a check of the conditions in the directions of the upcoming observations is possible. The FRAM concept is built upon experience gained with a similar device operated at the Pierre Auger Observatory. A working prototype of FRAM proposed for CTA is being built in Prague for extensive testing before deployment on site; first results and experiences with this prototype are presented.
Flasher and muon-based calibration of the GCT telescopes proposed for the Cherenkov Telescope Array  [PDF]
Anthony M. Brown,Thomas Armstrong,Paula M. Chadwick,Michael Daniel,Richard White,for the CTA consortium
Physics , 2015,
Abstract: The GCT is a dual-mirror Small-Sized-Telescope prototype proposed for the Cherenkov Telescope Array. Calibration of the GCT's camera is primarily achieved with LED-based flasher units capable of producing $\sim4$ ns FWHM pulses of 400 nm light across a large dynamic range, from 0.1 up to 1000 photoelectrons. The flasher units are housed in the four corners of the camera's focal plane and illuminate it via reflection from the secondary mirror. These flasher units are adaptable to allow several calibration scenarios to be accomplished: camera flat-fielding, linearity measurements (up to and past saturation), and gain estimates from both single pe measurements and from the photon statistics at various high illumination levels. In these proceedings, the performance of the GCT flashers is described, together with ongoing simulation work to quantify the efficiency of using muon rings as an end-to-end calibration for the optical throughput of the GCT.
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