High resolution spectrometer for atmospheric studies: Filling in observation of sky spectra absorption lines

High resolution spectrometer for atmospheric spectroscopy studies has been developed at the University of L’Aquila, Italy. The instrument that is mainly assembled for tropospheric hydroxyl radical (OH) long path studies, is used to study the sky spectra absorption lines during the setting up and preliminary tests. The system includes a double pass Echelle spectrometer with focal length of 2 m, a prism premonochromator to select the orders of the spectrometer and a CCD as detector. The nominal dispersion of the instrument is 0.4pm at 308nm, where there are the absorption lines of OH. During the study of the solar lines of Fe, Ni and Ca around 558.8nm the dispersion was of 0.8pm/pixel and the resolution of 1.5pm. Observations of direct solar spectrum and sky spectrum show a filling in of the sky lines due to rotational Raman scattering. The controversial solar angle dependence of the filling in sky absorption lines is discussed. Results of the laboratory tests and atmospheric measurements are also reported.