CTA Atmospheric Calibration Gaug, Markus
EPJ Web of Conferences,
01/2017, Letnik:
144
Journal Article, Conference Proceeding
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The main contribution to the systematic uncertainties of Atmospheric Imaging Cherenkov Telescopes (IACTs) currently stems from the uncertainty in the determination of atmospheric properties for a ...given moment of observation. This technique uses the atmosphere as a calorimeter of gamma-ray induced atmospheric air showers and measures the amount of Cherenkov light collected by large mirrors and focused towards a pixellized camera. Atmospheric conditions affect the measured Cherenkov light yield in several ways: the air-shower development itself, the variation of the Cherenkov angle with altitude and the loss of photons due to scattering and absorption of Cherenkov light. Although IACTs use to be located at astronomical sites, characterized by extremely clear atmospheric conditions, the local atmosphere should be continuously monitored, in terms of molecular density profiles, aerosol extinction profiles, and clouds. Moreover, a general understanding of the site and particularly aerosol climatology is desirable for a robust interpretation of the sensing instruments’ data. Here, the strategy of the Cherenkov Telescope Array (CTA) on atmospheric monitoring is presented, designed to ensure that the related systematic uncertainties are brought down by at least a factor of two with respect to current installations. Additionally, a more intelligent scheduling scheme is aimed at, where source observations are selected using dedicated auxiliary atmospheric monitoring instruments and taking into account the feasibility of a later analysis according to the underlying scientific case. This plan should reduce data loss during offline data selection and enhance the effective duty cycle of the CTA.
The Cherenkov Telescope Array (CTA) will bring a whole new insight to the gamma-ray Universe. In order to fulfill its performance requirements, we need to understand and correct the atmospheric ...effects that influence the acquired instrument data. One such systematic effect is due to the varying molecular density profile with time. We have studied such profiles for both CTA sites using publicly available historical data assimilation archives. Our study reveals that we can distinguish at least three differentiated seasonal periods at the northern site and at least two at the southern site, that allow to model the molecular part of the atmosphere using average profiles, as done with current Cherenkov telescope projects. Seasonal transitions are smoother at the southern site than at the northern one. Moreover, the latter shows a greater amplitude in density variations at an altitude of 15 km. We also explored deviations of the molecular profiles with respect to their mean values using a 5-years data set and concluded that they are always found within specifications.
A method for analyzing returns of a custom-made “micro”-LIDAR system, operated alongside the two MAGIC telescopes is presented. This method allows for calculating the transmission through the ...atmospheric boundary layer as well as thin cloud layers. This is achieved by applying exponential fits to regions of the back-scattering signal that are dominated by Rayleigh scattering. Making this real-time transmission information available for the MAGIC data stream allows to apply atmospheric corrections later on in the analysis. Such corrections allow for extending the effective observation time of MAGIC by including data taken under adverse atmospheric conditions. In the future they will help reducing the systematic uncertainties of energy and flux.
We discuss a calibration method for imaging atmospheric Cherenkov telescope arrays, based on the detection of cosmic rays. The focus lies on the monitoring of transmission of Cherenkov light in the ...atmosphere and on the relative calibration of telescope optical throughput efficiencies. We present an approach that addresses both issues by surveying and comparing trigger rates of telescopes in a stereoscopic configuration. A Monte Carlo feasibility study was conducted to explore dependencies of stereo trigger rates on the array layout and observing conditions of the Cherenkov Telescope Array (CTA). Analytical expressions for most of these dependencies have been found and implemented in an extension of the method of the Cherenkov Transparency Coefficient (CTC). In the investigated examples, the resolution of the method for the atmospheric and array calibration has been shown to be 4% and 4–7%, respectively.
Recently the readout of the MAGIC telescopes has been upgraded to a new system based on the Domino Ring Sampler version 4 chip. We present the analysis techniques and the signal extraction ...performance studies of this system. We study the behavior of the baseline, the noise, the cross-talk, the linearity and the time resolution. We investigate also the optimal signal extraction. In addition we show some of the analysis techniques specific to the readout based on the Domino Ring Sampler version 2 chip, previously used in the MAGIC II telescope.
•We perform studies of the DRS4 based readout of the MAGIC telescopes•Advanced pedestal subtraction procedure results in a stable baseline.•The noise of the system is below one photoelectron.•The calibration of the DRS4 time response gives time resolution of 0.2ns.•Linearity is very good up to saturation at 750phe.
We present the current knowledge and activities to assess the climatology of the “Observatorio del Roque de los Muchachos” (ORM), selected to host the Northern observatory of the CTA, with particular ...emphasis on molecular density profiles and aerosol extinction.
The IFAE/UAB Raman LIDAR for the CTA-North Gaug, Markus; Blanch, Oscar; Çolak, Merve Sıdıka ...
EPJ Web of Conferences,
2019, Letnik:
197
Journal Article, Conference Proceeding
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The IFAE/UAB Raman LIDAR project aims to develop a Raman LIDAR suitable for the online atmospheric calibration of the CTA. Requirements for such a solution include the ability to characterize aerosol ...extinction to distances of more than 20 km with an accuracy better than 5%, within time scales of less than one minute. The Raman LIDAR consists therefore of a large 1.8 m mirror and a powerful pulsed Nd-YAG laser. A liquid light-guide collects the light at the focal plane and transports it to the readout system. An in-house built polychromator has been characterized thoroughly with respect to its capability to separate effciently the different wavelengths (355 nm, 387 nm, 532 nm and 607 nm). It was found to operate according to specifications, particularly that light leakage from the elastic channels (532 nm and 355 nm) into the much dimmer Raman channels (387 nm and 607 nm) could be excluded to less than 2 × 10
-7
. We present here the status of the integration and commissioning of this solution and plans for the near future. After a one-year test period at the Observatorio del Roque de los Muchachos, an in-depth evaluation of this and the solutions adopted by a similar project developed by the LUPM, Montpellier, will lead to a final Raman LIDAR proposed to be built for both CTA sites.
The All Sky Camera (ASC) is a passive non-invasive imaging system for rapid night sky atmosphere monitoring. By design, the operation of the ASC will not affect the measurement procedure of the CTA ...observatory, for which we discuss its application in this report. The data collected should enable improved productivity and increased measurement time for the CTA observatory. The goal of ASC is to identify cloud position, atmosphere attenuation and time evolution of the sky condition, working within the CTA Central Calibration Facilities (CCF) group. Clouds and atmosphere monitoring may allow near-future prediction of the night-sky quality, helping scheduling. Also, in the case of partly cloudy night sky the cameras will identify the uncovered regions of the sky during the operation time, and define potential observable sources that can be measured. By doing so, a higher productivity of the CTA observatory measurements may be possible.
The analysis of ring images produced by muons in an Imaging Atmospheric Cherenkov Telescope (IACT) provides a powerful and precise method to calibrate the IACT optical throughput and monitor its ...optical point-spread function (PSF). First proposed by the Whipple Collaboration in the early 1990s, this method has been refined by the so-called second generation of IACT experiments: H.E.S.S., MAGIC, and VERITAS. We review here the progress made with these instruments and investigate the applicability of the method as the primary throughput calibration method for the different telescope types forming the future Cherenkov Telescope Array (CTA). We find several additional systematic effects not yet taken into account by previous authors and propose several new analytical methods to include these in the analysis. Slight modifications in hardware and analysis need to be made to ensure that such a calibration works as accurately as required for the CTA. We derive analytic estimates for the expected muon data rates for optical throughput calibration, camera pixel flat-fielding, and monitoring of the optical PSF. The achievable statistical and systematic uncertainties of the method are also assessed.
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