Astron. Astrophys. 363, 585-592 (2000)

2. Observations and data reduction

The observations were carried out with FEROS at the ESO 1.52-m telescope in La Silla. FEROS (Fiber-fed Extended Range Optical Spectrograph) is a bench-mounted fiber-linked echelle spectrograph located in the former Coudé room of the 1.52-m telescope. FEROS has no moving parts and is located in a temperature and humidity-controlled room, which allows for very stable operation.

The instrument is designed for high-dispersion spectroscopy with a resolution of , covering the spectral range from 3700-9200 Å in one exposure. FEROS is equipped with a 2k 4k 15µm pixel CCD. The high resolution is achieved by an image slicer (Kaufer 1998) which increases the spectral resolution by a factor of two.

FEROS uses two fibers simultaneously. One fiber contains the object spectrum. The second fiber can contain either the sky spectrum or the simultaneously exposed spectrum of a ThArNe-calibration lamp. The latter mode is intended for highest accuracy in radial velocity measurements and was not used in our observations. A more detailed description is given by Kaufer et al. (1997) and Kaufer and Pasquini (1998).

For a complete and fully automatic reduction of the data a special data reduction software was developed. It is implemented as a context in ESO-MIDAS. Fully automatic reduction for preview data is done on-line at the telescope. The reduction can include optimum extraction, following the method of Mukai (1990). Maximum signal-to-noise ratio () is achieved by choosing optimum weights for the summation of the pixel values. This procedure also allows removal of cosmic ray hits. The wavelength calibration is done by a global fit formula, which calibrates all echelle orders simultaneously. For details of the reduction procedure see Stahl et al. (1999).

The data of  Ori E were taken during November and December 1998, partly during the commissioning phase of FEROS. Note that the data from the commissioning phase are public and can be obtained from: http://www.lsw.uni-heidelberg.de/~akaufer/Feros/

 Ori C was observed in July and August of 1999. For  Ori E and  Ori C, exposure times of 15 min and of 6 min, respectively, were typically used. The of the extracted spectra depends on the wavelength and peaks at about 4500-5000 Å. In good conditions the typical peak ratio is 250 and 300 for  Ori E and  Ori C, respectively.

Many of the observations of  Ori E have been done during the comissioning phase of FEROS, and part of the spectra are of poor quality. The two worst spectra were omitted for this work. In total, 27 spectra of  Ori E were used; they are listed in Table 1.

Table 1. FEROS-spectra of  Ori E. The phases were calculated with the period of days and the epoch

14 spectra of  Ori C were obtained in 1999 and are listed in Table 2.

Table 2. FEROS-spectra of  Ori C. The phases were calculated with the period of days and the epoch

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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