Astron. Astrophys. 358, 233-241 (2000)

2. Observations and data reduction

2.1. Observations

CCD spectra of the sky, and Reticuli were obtained in September 1991 at the European Southern Observatory (ESO). The observations were carried out with the Coudé Echelle Spectrograph (CES) fed by the 1.40 m Coudé Auxiliary Telescope (CAT). The spectra comprised 5 regions, centered at 4370 Å, 4560 Å, 4860 Å (), 6110 Å and 6500 Å, covering 35-55 Å each. Complementary Reticon spectra, centered at 6240 Å and 6720 Å and covering 50 Å each, were observed with the same spectrograph and telescope in February 1992. The estimated S/N ratios for all these spectra ranged from 300 to 400. Their resolution was from 0.090 to 0.135 Å (resolving power R  50 000).

CCD spectra centered in the line (6563 Å), and covering 150 Å each, were obtained in 1994 and 1995 with the coudé spectrograph fed by the 1.60 m telescope of the Observatório do Pico dos Dias (CNPq/LNA, Brazil). These spectra had a S/N ratio of 200 and a resolution of 0.330 Å (resolving power R  20 000).

2.2. Data reduction and determination of equivalent widths

Bias extraction, flat fielding, linearization and wavelength calibration were carried out in the conventional way using the IRAF reduction package on a SUN workstation. Once corrected of Doppler shifts the spectra were normalized by fitting the mean flux in selected continuum windows, identified by comparison with the solar spectrum atlas of Kurucz et al. (1984), with low ( 4) order polynomials. The average internal deviation of the continuum fitting was 0.2%.

We have selected the stellar lines to be measured according to the compilation of Moore et al. (1966). Measurements of the equivalent widths were accomplished by least-squares gaussian fittings. The fitting of up to 4 gaussians simultaneously allowed us to successfully obtain equivalent widths of moderately blended lines, unresolved lines being rejected. We checked the accuracy of our measuments by comparing our solar equivalent widths with those of the atlas of Kurucz et al. (1984). The comparison (Fig. 1) is excellent, with a linear fit of high correlation (R = 0.996) and low standard deviation ( = 1.7 mÅ). No systematic deviations have been found.

Fig. 1. Comparison between sky equivalent widths (this work) and the Kurucz et al. (1984) solar atlas equivalent widths for all measured atomic lines. Dotted line is a linear fit.

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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