3 Reductions

All reduction steps have been performed with packages under IRAF. We followed essentially the method described by Way et al. (1998) with the following basic reductions steps.

All spectra are arranged side by side on the 2DF x-y-pixel image, the y direction being the direction of dispersion. Since the spectra have a strong curvature on the x-y-pixel plane, they have to be straightened first. We fitted a transformation of the plane in x direction by tracing the features in the combined image of the flatfield exposures. Similarily, the features in the combined calibration lamp exposure were fitted for the y transformation. All images were straightened by these two transformations. (This procedure uses the IRAF tasks identify, reidentify, fitcoords and transform in the noao.twodspec.longslit package.)

The extraction of the single spectra, tracing of the apertures, fitting of the flatfield, wavelength calibration with the two arc spectra, and subtraction of the sky spectra have been done with the dohydra package. The residual sky lines in the spectra were cleaned with the lineclean task. The typical rms error of the solution for the wavelength calibration is less than 0.3 Å.

Figure 1 shows 4 typical spectra of different signal-to-noise, with some reference lines indicated.

  

Figure 1: Typical spectra after subtraction of the sky spectra. The uppermost panel shows the nucleated dwarf elliptical NGC 1396. In the panel below the spectrum of the compact nucleus-like Fornax member CGF 1-4 is shown. In the third panel a typical emission line spectrum of a background spiral is plotted. The spectrum in the bottom panel has a signal-to-noise at the limit where velocity measurements are still feasible