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2 Observations and reduction

The spectroscopic observations were performed during dark time on the nights of February 24 to 26, 1995, using a REOSC spectrograph mounted on the f/8.48 Cassegrain focus of the CASLEO 2.15 m Ritchey-Chrétien telescope in El Leoncito (San Juan, Argentina). The seeing was between 2$\hbox{$^{\prime\prime}$}$and 3$\hbox{$^{\prime\prime}$}$. The detector was a Tektronix CCD $1024 \times 1024$ 24 $\mu$m pixels, corresponding each to 0$\hbox{$.\!\!^{\prime\prime}$}$26. The exposures were obtained through a slit of 3$\hbox{$.\!\!^{\prime\prime}$}$3 $\times$ 348$\hbox{$^{\prime\prime}$}$, on the focal plane, at the six positions indicated in Fig. 1. Five are at PA = 40$\hbox{$^\circ$}$, which was assumed to be the PA of the line of nodes, and one along the narrow northern arm, at a PA of 79$\hbox{$^\circ$}$. All positions were referred to the center of the galaxy. Instead of shifting the slit for each position, the slit was positioned first on the center and then the telescope was moved in order to bring the slit to the required position.

In total, there were seven exposures, two of them using a 300 lines mm-1 grating at an angle of 4$\hbox{$^\circ$}$50$\hbox{$^\prime$}$, over the wavelength range of $\lambda\lambda$3500-7000 (positions 3 and 5 in Fig. 1), and the other five with a 1200 lines mm-1 grating at an angle of 25$\hbox{$^\circ$}$53$\hbox{$^\prime$}$, covering the wavelength range of $\lambda\lambda$6200-6900 (positions 1, 2, 3, 4, and 6 in Fig. 1). The dispersions were 125 and 32 Å mm-1, the reciprocal dispersions 3.1 and 0.76 Å per pixel and the resolutions 10 and 2.5 Å for the 300 and 1200 lines mm-1 gratings, respectively. The usual procedure was followed for the observations, including the observation of the flat field, the comparison spectra and the dark current. The exposures were, in general, of 20 min and repeated till the total time indicated in Table 1 was reached for each position. The spectra were corrected for atmospheric extinction and galactic reddening (Burstein & Heiles 1984) and the flux was calibrated with stars from the catalog of Stone & Baldwin (1982). Data reduction of the spectra was made employing standard methods of the IRAF (developed by NOAO) reduction package.


  
Table 1: Observations

\begin{tabular}
{\vert ccc\vert}
\noalign{\smallskip}
\hline
\noalign{\smallskip...
 ...& 1200 & 60 \\  5 & 300 & 80 \\  6 & 1200 & 80 \\ \noalign{}
\hline\end{tabular}
a As given in Fig. 1.


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