  Direct Sun light 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 also needed 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 closed 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{\deg} mirrors in altazimuthal
set-up with a light sensor on the spectrometer, and to illustrate them with a
tracker developed for atmospheric research by our group.
