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4 Data reduction

The data were reduced by the observers and delivered to T. H. Dall as one dimensional spectra. Different reduction methods have been used as chosen by the observers. Parts of the McDonald data were given a first reduction by Horner, and later completed by Dall.

Nordic Optical Telescope

All basic reductions and extraction of one dimensional spectra were done using IRAF tools.

All spectra had bias subtracted after correcting for bad columns on the CCD. The scattered light between orders was removed before normalising the master flatfield. After flat-fielding, the orders were extracted and normalized with the Blaze function of the flatfield.

We chose to normalize the orders with the Blaze function of the flatfield instead of using the Blaze for the individual images, in order to use a constant normalization. This causes the spectral orders to be sloped because of the temperature difference between the flatfield lamp and the star. However, this does not influence the subsequent analysis, as no absolute spectral quantities are sought.

The NOT data did not contain an overscan strip, but this does not appear to be critical. Bias frames were taken at the beginning and end of each night, and no major drifts were found.

The spectra were not continuum normalized and no wavelength calibrations were done. The order containing H$\alpha$ is shown for a NOT spectrum in Fig. 3.

Thüringer Landessternwarte

The reduction was performed using MIDAS. After bias subtraction, correction for bad columns and flat-fielding, the orders were extracted and normalized with a Blaze function fitted to the spectra.

Wavelength calibrations were done using ThAr spectra. The MIDAS wavelength calibration routines were modified to give a better fit at the overlapping edges of the spectral orders.

Finally, all orders were merged into a single one-dimensional spectrum. An example of a spectrum is shown in Fig. 4.

Okayama Astrophysical Observatory

All reductions were performed using IRAF. First, bias and dark level were subtracted, before proceeding to flat-fielding with a master flatfield constructed from ten individual exposures.

The spectra were then aperture extracted and wavelength calibrated using Th-Ar spectra. An example of an extracted spectrum is given in Fig. 5.

Vainu Bappu Observatory

The various tasks for spectroscopic data reduction under the IRAF package were used for the data reduction.

All spectra were bias subtracted using a mean bias value and flat-field corrected. The spectra were extracted using the optimal extraction method of IRAF. Several Fe-Ne source spectra were obtained during the observations and an average source spectrum weighted according to the times of observation was used for the wavelength calibration.

No flux calibration has been applied. A spectrum can be seen in Fig. 6.

McDonald Observatory

All reductions were performed using IRAF tasks. First bias was subtracted, and bad column correction was applied. A master flatfield was constructed for each night and the orders normalized before the flat-fielding. Then the orders were extracted and normalized with the Blaze function of the flatfield (see discussion above for NOT data).

No flux calibrations or wavelength calibrations were performed. An example of one order, containing H$\alpha$ is given in Fig. 7.

Figure 3: One order from a spectrum taken from NOT. The red side is to the left. H$\alpha$ and some atmospheric bands are clearly visible.

Figure 4: The combined spectrum from TLS. Red side to the left.

Figure 5: Spectrum from OAO around H$\alpha$ line. Relatively high noise because of low count rate in high resolution.

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