For an experimentalist, it is always crucial to understand their detectors well. In astroparticle physics, the atmosphere itself is the detector, as that is where the extensive air showers, initiated by cosmic ray particles or gamma photons, develop. It is thus not surprising that the atmospheric conditions have to be thoroughly monitored. As the observation of atmospheric showers amounts to detection of faint light (be it fluorescence light at the Pierre Auger Observatory or Cherenkov light in the CTA project) from a distance of kilometres to tens of kilometres, one of the key parameters of the atmosphere is its transparency - or more specifically the amount of extinction of light between the point of its origin in the shower and the detector.
(top left) First CTA FRAM in Chile; top right: upgraded Auger FRAM in Argentina; bottom left: the second CTA FRAM on La Palma; bottom right: packing of the third CTA FRAM before its shipment to Chile.
While the molecular extinction, caused by scattering or absorption by individual molecules of atmospheric gases, is relatively stable in time and can be modelled using global meteorological models, the contribution of aerosols - solid or liquid particles suspended in the air, such as dust, smoke, fog or clouds - is highly variable and can influence for example the estimate of primary energy of a cosmic ray or gamma photon at the level up to tens of per cent. For these reasons, the atmospheric monitoring activities at the Institute of Physics focus on the monitoring of aerosols.
The golden standard for the determination of the aerosol contents of air is the measurement using the LiDAR, a system combining a powerful pulsed laser with a sensitive photodetector. Such measurement, however, produces copious amounts of light, which comes in quantities large enough to limit or even suspend the operation of an astroparticle experiment. Consequently, passive methods of aerosol measurement using natural light sources have an irreplaceable position in the field. These sources can be the Sun (obviously only during the day), the Moon (if sufficiently illuminated and high above the horizon) or the stars. The Sun and the Moon are used by a widely available device - the Sun/Moon Photometer for which we have been developing advanced methods of data processing, in particular with regard to the fast-changing brightness of the Moon.
The use of stars as a reference light source shows clear advantage in their availability during moonless nights and over the whole sky, while the disadvantages lie in the variety of physical properties of individual stars, inhomogeneous sky coverage of catalogues and most importantly in the small light flux of individual stars necessitating a very precise calibration of the measuring devices. The All-sky Camera system developed in the Joint Laboratory of Optics in Olomouc uses the simple binary detection of presence (or lack thereof) of stars on the whole sky at once in order to detect clouds. This information then can be used for real-time scheduling of observations as well as for the selection of data unaffected by clouds or even, in cooperation with an infrared LiDAR, for 3-dimensional characterisation of clouds and consequently for data calibration. The infrared LiDAR, also known as the Ceilometer, is being adapted in our group for use at astroparticle experiments.
Small robotic “FRAM” telescopes observe a smaller part of the sky, but they allow the measurement of the apparent brightness of stars which is then compared with catalogues in order to determine the extinction; a FRAM telescope can measure hundreds to thousands of stars in a single image. When a series of measurements are taken in quick succession in different altitudes above the horizon, a very precise value for the overall aerosol content of the atmosphere is obtained; when the FRAM follows a single field of view continuously (such as it will do with the CTA field of view), it can identify any changes in the conditions in that directions and “order” a full vertical profile to be taken by a LiDAR. At the Institute of Physics, we develop the robotic systems of the telescopes in order to facilitate long-term operation with minimal supervision and maintenance as well as methods for automated processing of data from a large amount of observed stars while controlling a large gamut of systematic effects so that the precision is better than 1 % of attenuated light.
Installation of first atmospheric devices at the southern CTA observatory in Chile, personnel of FZU at work.
