For both visible and NIR, particular care has been paid to
the correction for atmospheric extinction, recording spectra
of different solar analog stars at different airmasses during
all the nights. At the end all the solar analogues showed negligible differences. As proof of the accuracy of our method
we computed the ratio between the solar analogues’ spectra,
obtaining a low noise, almost a constant curve. In the visible, the ratio shows a P-Cygni profile around 0.76 µm, due
to the atmospheric O2, but this artificial feature is restricted
to a narrow wavelength interval and does not affect the behavior at other wavelengths. The maximum deviation from 1
is of about a few percent. As for the NIR, the ratio shows the
very noisy region of deep atmospheric absorptions, which
are restricted to the intervals 1.1–1.15 and 1.3–1.5 µm. Outside these windows the curves differ from 1 only for a few
percent.