Atmospheric monitoring and array calibration in CTA using the Cherenkov Transparency Coefficient

Stefanik, Stanislav; Reyes, R. de los; Nosek, D.

doi:10.22323/1.301.0856

Cited by 4 publications

(4 citation statements)

References 7 publications

(13 reference statements)

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“…In previous publications, a phenomenological approach was used to link the measured trigger rate with properties of the atmosphere [e.g. 6,7]. The goal of the presented study is to find a physics driven fit model to describe the measured trigger rates for hadron induced showers (background) and gamma induced events (signal) as a function of the observed zenith angle θ.…”

Section: Introductionmentioning

confidence: 99%

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Bretz

2019

Astroparticle Physics

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A: The rate of extensive air-showers observed with imaging air-Cherenkov telescopes is zenith angle dependent. This effect originates from the increasing geometrical distance of the observed shower to the telescope with increasing zenith distance. This paper investigates how this alters the observed image and how this affects the trigger rate as a function of zenith angle. The discussed effects include the change of Cherenkov light yield, of absorption in the atmosphere, of photon density at the aperture and of the image size at the focal plane of the telescope. Based on a simple model for the atmosphere and well-known first principles on the development of extensive air-showers, the zenith angle dependence is expressed analytically. The assumption that most light is emitted from the shower core and mathematical approximations allow to derive an analytical expression describing the zenith angle dependence well with only three free parameters which are directly linked with the underlying physics. This suggests further investigations about how these fit parameters are linked to the properties of the atmosphere and the instrument. Using data published by the First G-APD Cherenkov Telescope, a good match of the fit functions with the data is obtained. For the trigger rate of cosmic rays, the obtained parameters are consistent with the naive expectation.

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Section: Introductionmentioning

confidence: 99%

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Bretz

2019

Astroparticle Physics

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“…Lidars, FRAM and AERONET Sun/Moon photometer). The preliminary results of a feasibility study for the application of the CTC in CTA are reported in a separate contribution in these proceedings [21].…”

Section: Further Devices and Methods In Developmentmentioning

confidence: 99%

Atmospheric calibration of the Cherenkov Telescope Array

Ebr¹,

Bulik²,

Font³

et al. 2017

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

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The atmospheric monitoring devices for the planned calibration system of the Cherenkov Telescope Array (CTA) are undergoing intensive development, prototyping and testing. The All-Sky Cameras, the Sun/Moon Photometers and the FRAM telescopes have been gradually deployed at the future CTA sites with the primary goal of site characterization, simultaneously allowing the assessment of their operational reliability in realistic environmental conditions. All three devices have shown the ability to work smoothly in both the extreme dryness and the large temperature variations of the southern site as well as in the occasional adverse weather during winter months at the northern site. The target availability of 95% of time has not yet been reached mostly due to minor hardware failures that have proven difficult to fix due to the remoteness of the installation in the absence of the future CTA infrastructure. The experience gathered during the prototype operations will contribute to the improved reliability of the final instruments. The Raman LI-DARs, described in separate proceedings of this conference, and the infrared Ceilometer, ready for testing in Prague, will complement the set of atmospheric calibration devices in near future. The final operational procedures for the atmospheric calibration of the CTA during its operation are being finalized foreseeing the use of the All-sky Cameras and the Ceilometer for the monitoring of clouds over the whole sky and the LIDARs and FRAMs for precision measurements of the atmospheric transmission as a function of altitude and position within the field-of-view of the CTA array.

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“…Additionally to that, redundant information is obtained for calibration and cross-checks by photometric measurements of reference stars using a sophisticated, portable small-aperture multi-pixels photon detector, the UVScope and the UVSiPM [58,59] and the "Cherenkov Transparency Coefficient" (CTC) calculated directly from science data [60].…”

Section: Aerosol Characterization Of the Observed Field-of-view By Ctamentioning

confidence: 99%

CTA Atmospheric Calibration

Gaug

2017

EPJ Web Conf.

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Abstract. 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.

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Atmospheric monitoring and array calibration in CTA using the Cherenkov Transparency Coefficient

Cited by 4 publications

References 7 publications

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Atmospheric calibration of the Cherenkov Telescope Array

CTA Atmospheric Calibration

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