Astron. Astrophys. 319, 630-636 (1997)

2. Observations

Two echelle spectra were obtained by G. Mathys at the European Southern Observatory in June 1992 with the ESO Multi Mode Instrument mounted on the New Technology Telescope. The Red Medium Dispersion spectroscopy mode was used in combination with the CCD #18 (Thomson 1024x1024 pixels). The echelle grating #10 with two different cross-disperser grisms covered the entire spectral range from 4200 to 8100 Å. Grism #6 allows to observe 22 well separated orders in a wavelength range from 6200 to 8100 Å. Grism #5 covers 46 orders, ranging from 4200 to 6400 Å. The spectral range of each order varies from 40 to 90 Å for the blue exposure, and from 90 to 120 Å for the red exposure. The spectral resolution in the blue region is , and in the red.

The typical signal-to-noise ratio per pixel is about 200 in the continuum. To avoid the influence of the decreased signal-to-noise ratio far from the blaze, each order was trimmed such that two consecutive orders overlap only by approximately 10 to 20 Å.

Echelle spectra of A-type stars pose the well-known problem of defining the continuum in the relatively large spectral interval which is covered by hydrogen line wings. Echelle orders adjacent to those which contain the hydrogen lines were therefore interpolated to determine the trend in the blaze function and continuum.

All reductions of the observations to one continuous spectrum and the normalization to the continuum were done within the echelle package of IRAF. The normalization to the continuum was independently checked and confirmed by T. Ryabchikova using PC-IPS (Piskunov & Smirnov 1994).

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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