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CCD observations and data reduction

The observations in 1996 were carried out with a new CCD-detector in two different set-ups. In February 1996 we used B0 grating (300 grooves/mm) with the wavelength coverage of 3600 - 7000Å and a dispersion of 5.2 Å/pixel and B1 grating (600 grooves/mm) with the interval 3600 - 5400Å and a dispersion of 2.6 Å/pixel. For an experimental set-up of the CCD-detector there was a significant vignetting which limited the actual wavelength range for both gratings. The spectra were obtained through a long slit of 1''-2'' width and 45'' length. For the October 1996 run the CCD set-up allowed the coverage of the range 3600 - 6000 Å with the dispersion 2.3 Å/pixel and grating B1. In these observations we used the registration system NICE under MIDAS (Kniazev & Shergin [1995]). As in the case with IPCS observing mode the exposure time for both set-ups did not usually exceed 2-3 min which was enough to recognize the strongest emission lines from the low S/N ($\la$ 10) spectra.

Ten exposures have been obtained both in the evening and morning twilight to get the estimate of CCD detector's flat field. All primary reduction of CCD spectra was conducted in the standard way, using MIDAS context Long (94NOV version). 2-D CCD images after the correction for the dark noise, debiasing and sky-substraction were transformed to linear wavelength scale and converted to 1-D spectra with the following correction for the atmospheric extinction and standard flux calibration. The same as for the IPCS-mode He-Ne-Ar source was used to build up the dispersion curve. Spectrophotometric standards have been observed minimum twice during the night to allow the correction for the spectral response of the system.

The information on the set-up of the spectral equipment, used wavelength range, spectral resolution and the number of the observed objects for each run is shown in Table 1.