Regular articleDesign and implementation of a heliostat for atmospheric spectroscopy
Section snippets
Introduction: context and state-of-art
Optical laser spectrometry is a tool based on the Beer-Lambert law and widely used for atmospheric studies, either in situ — for example CEAS (Cavity Enhanced Absorption Spectroscopy) [1] and photoacoustic spectroscopy [2] — or remotely — for example LIDAR (Light Detection And Ranging) [3] and heterodyne spectrometry [4]. The heterodyne spectroscopy is a passive ground-based technique particularly used for atmospherical [5], [6] and astrophysical studies [7]. In the atmospherical case, a part
Optical set-up
The heliostat or suntracker that we built is composed of two plane mirrors and with elliptical section (see Fig. 1). They are inclined at 45 relative to the vertical of the location. adjusts the zenithal angle and the set ensures the azimuthal rotation. The mirrors are connected to motorized rotation stages. It is split into two paths by a BaF2 beamsplitter (reflection of per side). The reflected beam is focused on a Lateral Effect Photodiode (LEP, PDP90A - Thorlabs),
Calculation of solar positions
In the case of a perfectly oriented instrument, the suntracking can be done by calculating its positions during the day. A Matlab program using the algorithm of Jean Meeus is used [23], [24]. Zenithal and azimuthal angles of the Sun are obtained in a topocentric coordinate system. The calculation is made from the geographical coordinates of the tracker, the date, time and offset relative to Coordinated Universal Time (UTC). The zenithal angle is measured relative to the local vertical, that
Servo loop
The servo system consists in generating an error signal for correcting the rotation of the mirrors through a feedback loop. This is usually associated with a PID to optimize the response time. Generating the error signal can typically be made by a quadrant photodiode [14], [26], [27] or a camera [15]. A LEP (PDP90A – Thorlabs) which has an intermediate comportment was chosen. It enables to determine the position of the Sun’s image on its surface, regardless of its form or power distribution,
Atmospheric spectra
Down the skylight, solar beam reaches a diameter. A 3” diameter part of the beam is taken and its size is reduced by a factor 4.5 by the use two off-axis parabolic mirrors. This beam shaping is suitable for direct injection, ultimately, in a heterodyne spectrometer currently developed at our laboratory from optical diameter of 1”.
In order to test the heliostat performances for spectroscopy, the system is also adapted to inject the solar beam in a Fourier Transform Infrared spectrometer
Conclusion
The heliostat presented here operates without loss of control of the servo loop as it approaches the zenith. It enables to track the Sun for a whole day regardless of the time of year (measurements made at differents times of the year). The servo-control method developed in this work is effective enough to collect a solar beam and to stabilize it with an total angular error of . According to our knowledge, this stability is better than the other suntrackers described in the
Conflict of interest
The authors declared that there is no conflict of interest.
Acknowledgements
This work was funded by the ANR ASTRID Project #ANR 12-ASTR-0028 called QUIGARDE (2013–2016). The authors are also indebted to the Délégation Générale de l’Armement and the Centre National de la Recherche Scientifique for financial support of the PhD of Marie-Hélène Mammez.
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