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Register a new userCalibration of the CAT Telescope
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Authors
Frederic Piron
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Due to the lack of test-beams in ground-based gamma-ray astronomy, detector calibration has been a major challenge in this field. However, with the use of Cherenkov ring-images due to cosmic-ray muons and of strong gamma-ray signals, the CAT telescope could be rather well monitored and understood. Here we present a few oustanding aspects of this work.
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We report here observations of the active galactic nucleus Mrk 501, at energies above 250 GeV carried out with the CAT atmospheric imaging telescope from March 1997 to Autumn 1998. This source was in a high state of activity at several different wavelengths in 1997, and the observed flux at TeV energies has been seen to change by a factor of ~ 20 from from 1995 and 1996 fluxes. CAT observations also indicate a curved spectrum at TeV energies, and a correlation between the gamma-ray intensity and the spectral hardness. The temporal variability and the TeV spectral properties are examined.
The ANTARES deep-sea neutrino telescope comprises a three-dimensional array of photomultipliers to detect the Cherenkov light induced by upgoing relativistic charged particles originating from neutrino interactions in the vicinity of the detector. The large scattering length of light in the deep sea facilitates an angular resolution of a few tenths of a degree for neutrino energies exceeding 10 TeV. In order to achieve this optimal performance, the time calibration procedures should ensure a relative time calibration between the photomultipliers at the level of about 1ns. The methods developed to attain this level of precision are described.
We present an analysis of polarimetric observations of standard stars performed over the period of more than three years with the RINGO3 polarimeter mounted on the Liverpool Telescope. The main objective was to determine the instrumental polarisation of the RINGO3 polarimeter in three spectral energy ranges: blue (350--640~nm), green (650--760~nm) and red (770--1000~nm). The observations were conducted between 2012 and 2016. The total time span of 1126 days was split into five epochs due to the hardware changes to the observing system. Our results should be applied to calibrate all polarimetric observations performed with the RINGO3 polarimeter.
The Milagro detector is an air shower array which uses the water Cherenkov technique and is capable of continuously monitoring the sky at energies near 1 TeV. The detector consists of 20000 metric tons of pure water instrumented with 723 photo-multiplier tubes (PMTs). The PMTs are arranged in a two-layer structure on a lattice of 3 m spacing covering 5000 $m^2$ area. The direction of the shower is determined from the relative timing of the PMT signals, necessitating a common time reference and amplitude slewing corrections to improve the time resolution. The calibration system to provide these consists of a pulsed laser driving 30 diffusing light sources deployed in the pond to allow cross-calibration of the PMTs. The system is capable of calibrating times and the pulse-heights from the PMTs using the time-over-threshold technique. The absolute energy scale is provided using single muons passing through the detector. The description of the calibration system of the Milagro detector and its prototype Milagrito will be presented.
We present here results from large zenith-angle observations with the CAT atmospheric Cherenkov imaging telescope, based on data taken on the Crab Nebula and on the blazar Mk501 from 1996 onwards. From Monte Carlo simulations, the threshold energy of the telescope is expected to vary from about 250 GeV at zenith to about 2 TeV at a zenith angle of 60 degrees. The lower source-fluxes due to the increased threshold energy are partly compensated for by an increase in the effective collection area at large zenith angles, thus allowing a significant extension of the dynamic range of the CAT telescope, with a tolerable loss in sensitivity. We discuss the implications for source detection and energy spectrum measurements.
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