Abstract
Direct sunlight absorption by trace gases can be used to quantify them and investigate atmospheric chemistry. In such experiments, the main optical apparatus is often a grating or a Fourier transform spectrometer. A solar tracker based on motorized rotating mirrors is commonly used to direct the light along the spectrometer axis, correcting for the apparent rotation of the Sun. Calculating the Sun azimuth and altitude for a given time and location can be achieved with high accuracy but different sources of angular offsets appear in practice when positioning the mirrors. A feedback on the motors, using a light position sensor close to the spectrometer, is almost always needed. This paper aims to gather the main geometrical formulas necessary for the use of a widely used kind of solar tracker, based on two 45° mirrors in altazimuthal set-up with a light sensor on the spectrometer, and to illustrate them with a tracker developed by our group for atmospheric research.
Highlights
Spectroscopic analyses of direct incident sunlight are commonly used in atmospheric research.Such experiments make use of the Sun as a light source to quantify molecular absorptions in Sensors 2012, 12 the atmosphere and retrieve trace gas abundances
Stratospheric ozone [1] and greenhouse gases [2] are routinely measured with this technique from ground-based Fourier transform infrared (FTIR) spectrometers, e.g., within the Network for the Detection of Atmospheric Composition Change (NDACC, http://www.ndacc.org/)
A 4-quadrant photodiode is used as a position sensor for a closed-loop control of the mirrors position once their positioning towards the Sun has been set with enough accuracy, i.e., once the Sun’s image is visible by the photodiode. This altazimuthal setup is used with FTIR systems, e.g., in Kiruna [10] and Park Falls, Wisconsin [11]; it has been installed in Harestua to replace the equatorially mounted system [12]
Summary
Spectroscopic analyses of direct incident sunlight are commonly used in atmospheric research. A 4-quadrant photodiode is used as a position sensor for a closed-loop control of the mirrors position once their positioning towards the Sun has been set with enough accuracy, i.e., once the Sun’s image is visible by the photodiode This altazimuthal setup is used with FTIR systems, e.g., in Kiruna [10] and Park Falls, Wisconsin [11]; it has been installed in Harestua to replace the equatorially mounted system [12]. It is first necessary to characterize the field-of-view (FOV) of the 4-quadrant diode in the considered optical design This serves two purposes: determining the accuracy needed for the ephemeris’s algorithm and making sure this FOV is larger than the Sun’s apparent diameter (9 mrad).
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