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Astron. Astrophys. 335, 985-990 (1998)

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2. Observations and reduction

The observations and reduction of medium resolved spectra are described in Moehler et al. (1990a, PG 1323-086) and Conlon et al. (1993a, PG 1704+222). The higher resolved data were obtained with the Cassegrain Twin Spectrograph of the 3.5m telescope at the Calar Alto observatory, using a slit-width of [FORMULA] and a mean dispersion of 18 Å/mm in the blue and of 36 Å/mm in the red channel. Both channels were equipped with Tektronix CCDs with 1024 [FORMULA] 1024 pixels of 24 µm size. Read-out-noise and gain were 9 e- and 4.5 e-/ADU in the blue and 8 e- and 4.5 e-/ADU in the red channel. With this setup we covered the wavelength range 3850 Å - 4710 Å in the blue channel with two exposures and in the red channel we used the wavelength range 5850 Å - 6720 Å. The mean FWHM of the calibration lines was about 0.9 Å and 1.8 Å for the blue and red channel, respectively, and represents the typical spectral resolution of the system.

The observations and data reduction were performed as described by Moehler et al. (1995) except that we did not use any optimal extraction algorithm, as the spectra were tilted on the CCD. Instead we summed up the rows of each spectrum after sky-subtraction and tried to get rid of cosmic events by comparing several spectra of the same object. Finally the spectra were corrected for the Doppler shifts and coadded. The resulting mean radial velocities are listed in Table 5. To give an idea of the quality of our spectra we show part of them in Fig. 1.

[FIGURE] Fig. 1. Part of the blue spectra of PG 1323-086 and PG 1704+222 . Those spectral lines that could be identified are marked.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998