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Astron. Astrophys. 328, 219-228 (1997)

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2. Observations and data reduction

We have collected a total of 133 spectra in the blue wavelength region and 145 in the red, simultaneously recorded with the fiber fed Echelle spectrograph HEROS (Heidelberg Extended Range Optical Spectrograph) at the ESO 50 cm telescope. The light path is divided into a blue and red path after the Echelle grating, then cross-dispersed with a grating for each channel, and finally, the spectrum is recorded on two CCDs. The total wavelength coverage of the instrument is from 3500 Å to 8600 Å. Because of the dichroic beam splitter there is a gap of about 250 Å , centered at 5700 Å. During an observing night typically every 2 hours lamp spectra are recorded for flat fielding and wavelength calibration. The two pixel resolution is [FORMULA] with nominal accuracy and stability of the wavelength calibration of 0.35  [FORMULA] RMS (Kaufer et al. 1997). To verify these numbers in practice we have measured the radial velocities of interstellar Ca II   [FORMULA] and Na I   [FORMULA]. We find that the scatter in the red channel agrees with the nominal accuracy of the instrument with systematic drifts below [FORMULA] [FORMULA]. In the blue path the radial velocity drifts between 13.6 [FORMULA] and 14.5 [FORMULA] with a RMS deviation of 0.5 [FORMULA] (Fig. 1). Thus, our observations are more accurate than 1 [FORMULA].

[FIGURE] Fig. 1. Observed radial velocities of the interstellar Ca II   [FORMULA]. The dashed line indicates a drift of the wavelength calibration and the error bar represents the RMS deviation of the data set. The epoch is given by JD-2,400,000.5.

The signal to noise ratio of the spectra is [FORMULA] at 3600 Å and [FORMULA] at 6000 Å. The Echelle ripples are removed with the flat-field lamp response. Remaining ripple signatures are of the order of 1%. The spectra are normalized by a spline fit through selected wavelength points, roughly separated by 250 Å. These points are chosen to be free of emission lines but in the blue wavelength region, we probably do not reach the true continuum level.

The data were obtained in two observing runs from April 13 to June 2, 1995 and January 18 to April 30, 1996 (JD 2 449 821 to JD 2 449 871 and JD 2 450 101 to JD 2 450 204). We observed [FORMULA]  Velorum once a night with only few gaps. The data reduction was carried out by OS, AK, and TR based on a specially adapted version of the ESO-MIDAS Echelle context (Stahl et al. 1995).

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998