For the H-alpha region, some time it is necessary to correct spectra for the occurrence of telluric water vapor lines (H2O). For this purpose I have adapted a model of atmospheric spectral lines (GEISA date base). The synthetic atmosphere is adjusted semi-automatically to the observational data (FWHM, spectral shift, transmittance of the atmosphere), and finally the star spectrum is divided by the atmospheric model.
In red, the telluric H2O line around H-alpha line. In blue, a typical spectral resolution fit according to the real spectrum (parametric gaussian convolution of the red figure).
Enlargement of the H-alpha region. Red curve, the typical atmospheric transmission for a resolution of 0.1 A. Blue curve, same but for a spectral resolution of 2 A.
Red, the spectra of the Be-star Delta Scorpius (very low above the horizon - air mass of 3.1). The H-alpha emission is clear but also the H2O lines. Blue, the atmospheric model fitted to the Delta Sco spectra (note the good correlation). Green, the resulting Delta Sco spectrum corrected for the atmospheric transmission. The reason of residual is induced by the difficult to estimate the continuum baseline for this problematic star (remenber the high value of air-mass).
Comparison
of the unprocessed spectrum (up) and the processed spectrum (down).
This type of atmospheric correction is some time important to obtain
scientic valuable results.