3 Observations

Figure 1: The particular raw spectrum P as observed with a large CCD camera at VTT, Tenerife. The grey-scale bar gives relative intensities expressed in ADU (Analog-Digital Units).

We will demonstrate the full procedures of precise reduction of CCD spectra and spectral line characteristics determination using spectra taken at the VTT with an echelle spectrograph described by Schröter et al. (1985). The observations were performed at the VTT on April 28, 2000 with two $2048\times2048$ XEDAR CCD cameras (see Table 1). The observing project included observations in two spectral regions simultaneously. The spectra were exposed with a high cadence of 3 s for several hours. Here, we will deal with the observations of the spectral range (roughly $\sim$3 Å wide), centred on the Fe II 6456.396 Å line.

A quiet area near the solar disc centre was observed. More than 1000 exposures of spectra, flat-field source frames and background frames were collected. The width of the entrance slit of the spectrograph was 100 $\mu$m. The exposure time of every particular raw spectrum P as well as of the flat-field source frame F and background frame G was 1.0 s. The binning mode was switched to $2\times2$, it means that the final spectrum has 1024 pixels in both the wavelength and spatial directions. The line Fe II 6456.396 Å was observed in the 35th order. Thus, one pixel corresponds to 3.097 mÅ in the wavelength direction and one spatial step corresponds to 0.125 arcsec.

The typical CCD raw spectrum P taken at the spectrograph of the VTT using the large CCD camera is shown in Fig. 1, where X denotes the wavelength direction and Y denotes the space direction along the spectrograph slit.