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Atmospheric Monitoring for High Energy Gamma Ray Cherenkov
Telescopes Based on HSRL: Development of High Accuracy
Non-Invasive Etalon Characterization Techniques
S. Maltezos*, E. Fokitis, V. Gika, N. Maragos, E. Koubli, G. Koutsourakis
Abstract
In contemporary experiments for studying the high energy gamma rays by using
Cherenkov telescopes, like MAGIC, HESS and CTA, the atmospheric monitoring is of
high importance. The UV Cherenkov radiation, emitted during the extended air
showers, is scattered and partially absorbed in the atmosphere. In the event
reconstruction procedure the above effects have to be taken into account. A High
Spectral Resolution Lidar (HSRL) prototype in bistatic mode is being designed to use
a 120 mW CW diode SLM laser at 532 nm and two Fabry Perot etalons in the
receiver telescope. The two channels correspond to molecular and aerosol scattering
signal respectively. The receiver telescope is based on a 370 mm massive parabolic
mirror with /8 polishing quality and a prototype etalon for aerosol channel with
spacer length 100 mm made of Zerodure and dielectric mirrors with reflectivity 97 %
in the near UV range. The development of this HSRL prototype could allow us to
perform high accuracy atmospheric monitoring aiming also to determine the aerosol
phase function. In this work we present its design details and the first laboratory tests
of the receiver by means of optical alignment and collection efficiency. In this frame,
we also present and propose a complete, novel and non-invasive characterization
technique of the etalons to be used for the HSRL. The main idea is to determine the
spacer thickness of the etalons we are planning to use with the highest accuracy of
the order of nm scale. This can be done using two or three well known wavelengths
corresponding to coherent spectral lines. The procedure of our method is based on
the following particular steps:
a) We determine of the particular excess fractions  i of the corresponding
interference fringe pattern system with high accuracy, using a two-dimensional chisquare non-linear direct fit method.
b) We lay out the congruent or modular-like equations for 2 (or 3) of the known
wavelengths and investigate the optimum solution for the etalon spacer thickness
using modulo operators. In this method we use a multidimensional orthogonal
Excess Fraction Space (EFS) with  i used as bases.
The experimental technique for selecting the particular spectral lines is that of
crossing the etalons with a 1-m focal length stigmatic UV high-spectral resolution
spectrograph. The well-known triplet line of Hg at 365 nm is used in the
characterization procedure. Using the accurate values of etalon spacer thicknesses
we are plan to measure the absolute instantaneous wavelength of the dominant
longitudinal mode of the laser emitter using the above method, as well as, to study its
temporal variance (Allan’s variance type).
*
Corresponding author. E-mail: [email protected]