2 H$\alpha$ observations and reduction

Spectroscopic observations have been performed with the échelle spectrograph at the 91-cm telescope of Catania Astrophysical Observatory - M. G. Fracastoro station (Serra La Nave, Mt. Etna) - in 1997, 1998, and 1999.

The spectrograph is fed by the telescope through an optical fiber (UV - NIR, $200~\mu$m core diameter) and is placed in a stable position in the room below the dome level. Spectra were recorded on a CCD camera equipped with a front-illuminated CCD of $800 \times$ 1152 pixels (pixel size of $22.5~\mu$m) in 1997-98 and with a thinned back-illuminated SITe CCD of $1024\times$1024 pixels (size $24\times 24~\mu$m) in 1999. With the first detector we were able to record four orders in each frame, spanning from about 6050 to 6650 Å, and to cover nearly completely each échelle order. With the second one we could record five échelle orders, starting from 5850 Å. The echelle cross-configuration yields a resolution of about 14000, as deduced from the FWHM of the lines of the Th-Ar calibration lamp.

The average signal-to-noise ratio (S/N) obtained at the continuum near the H$\alpha$ line was about 150.

The data reduction was performed by using the ECHELLE task of IRAF package following the standard steps: background subtraction, division by a flat field spectrum given by a halogen lamp, wavelength calibration using the emission lines of a Thorium-Argon lamp, normalization through a polynomial fit to the continuum.