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Register a new userThe Atmospheric Monitoring Strategy for the Cherenkov Telescope Array
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Authors
M. K. Daniel
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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 Cherenkov 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.
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Reconstruction of energies of very-high-energy gamma-rays observed by imaging atmospheric Cherenkov telescopes is affected by changes in the atmospheric conditions and the performance of telescope components. Reliable calibration schemes aimed at these effects are necessary for the forthcoming Cherenkov Telescope Array (CTA) to achieve its goals on the maximally allowed systematic uncertainty of the global energy scale. A possible means of estimating the atmospheric attenuation of Cherenkov light is the method of the Cherenkov transparency coefficient (CTC). The CTC is calculated using the telescope detection rates, dominated by the steady cosmic ray background, while properly correcting for the hardware and observational conditions. The coefficient can also be used to relatively calibrate the optical throughput of telescopes on the assumption of homogeneous atmospheric transparency above the array. Using Monte Carlo simulations, we investigate here the potential of the CTC method for the atmospheric monitoring and telescope cross-calibration at the CTA array in the southern hemisphere. We focus on the feasibility of the method for the array of telescopes of three sizes in different observation configurations and under various levels of atmospheric attenuation.
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.
We present the current development of the Monitoring, Logging and Alarm subsystems in the framework of the Array Control and Data Acquisition System (ACADA) for the Cherenkov Telescope Array (CTA). The Monitoring System (MON) is the subsystem responsible for monitoring and logging the overall array (at each of the CTA sites) through the acquisition of monitoring and logging information from the array elements. The MON allows us to perform a systematic approach to fault detection and diagnosis supporting corrective and predictive maintenance to minimize the downtime of the system. We present a unified tool for monitoring data items from the telescopes and other devices deployed at the CTA array sites. Data are immediately available for the operator interface and quick-look quality checks and stored for later detailed inspection. The Array Alarm System (AAS) is the subsystem that provides the service that gathers, filters, exposes, and persists alarms raised by both the ACADA processes and the array elements supervised by the ACADA system. It collects alarms from the telescopes, the array calibration, the environmental monitoring instruments and the ACADA systems. The AAS sub-system also creates new alarms based on the analysis and correlation of the system software logs and the status of the system hardware providing the filter mechanisms for all the alarms. Data from the alarm system are then sent to the operator via the human-machine interface.
The current generation of Cherenkov telescopes is mainly limited in their gamma-ray energy and flux reconstruction by uncertainties in the determination of atmospheric parameters. The Cherenkov Telescope Array (CTA) aims to provide high-precision data extending the duty cycle as much as possible. To reach this goal, it is necessary to continuously and precisely monitor the atmosphere by means of remote-sensing devices, which are able to provide altitude-resolved and wavelength-dependent extinction factors, sensitive up to the tropopause and higher. Raman LIDARs are currently the best suited technology to achieve this goal with one single instrument. However, the synergy with other instruments like radiometers, solar and stellar photometers, all-sky cameras, and possibly radio-sondes is desirable in order to provide more precise and accurate results, and allows for weather forecasts and now-casts. In this contribution, we will discuss the need and features of such multifaceted atmospheric calibration systems.
The CTA is the next generation of ground based very high energy gamma ray Imaging Atmospheric Cherenkov Telescopes. Since observations with this technique are affected by atmospheric conditions, an accurate knowledge of the atmospheric properties is fundamental to improve the precision and duty cycle of the CTA. Measurements of absorption and scattering properties of the atmosphere due to aerosols and molecules can be used in the event reconstruction or in MODTRAN, an analytical code designed to model the propagation of electromagnetic radiation. MODTRAN output is used as an input for the air shower simulation and Cherenkov light production, giving the optical depth profiles that together with the refractive index allow the proper simulation of the gamma ray induced signals and a correct measurement of the primary energy from the detected signals. The ARCADE Raman Lidar will be used for the on site characterization of the aerosol attenuation profiles of the UV light. The collected data will be used in preparation for the full operation of the array, providing nightly information about the aerosol properties such as the vertical aerosol optical depth and the water vapour mixing ratio with an altitude resolution better than 100 m from about 400 m to 10 km above ground level. These measurements will help to define the needs for Monte Carlo simulations of the shower development and of the detector response. This instrument will also be used for the intercalibration of the future Raman Lidars that are expected to operate at the CTA sites. This contribution includes a description of the ARCADE Lidar and the characterization of the performance of the system. The system is expected to be shipped to the northern site of the CTA (La Palma) before the end of 2017, to acquire data locally for 1 year before being moved to the southern site (Chile).
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