1) It is essential to apply de-stretching (i.e. correction of the curvature and inclination of the spectral lines) before flat-fielding instead of after it, since the determined spectral line characteristics will have significant errors.
2) Only the precise reduction sufficiently suppresses the noise coming from the various components of the flat-field matrix M.
3) The most noisy component of the raw spectrum is the flat-field caused by the defects of the slit and by dust on the slit of the spectrograph.
4) Splitting of the flat-field matrix into several components and appropriate shifts of them should be applied in the case when the flat-field conditions were significantly changed during the observing run. In particular, this should be applied if the particular spectrum was taken much later than the flat-field frames.
5) The continuum intensities and line centre intensities are the spectral characteristics most sensitive to the temporal changes in the flat-field conditions. 6) Precise flat-fielding is essential for correct reduction of the large solar CCD spectra, because the inaccuracy remaining from insufficient reduction approaches will still affect all results based on the analysis of the profiles of spectral lines.
Acknowledgements
The VTT is operated by the Kiepenheuer-Institut für Sonnenphysik, Freiburg, at the Observatorio del Teide of the Instituto de Astrofisica de Canarias. Part of this work was supported by the Slovak grant VEGA 2/7229/20 and German DFG grant No. 436 SLK 113/7. A.H., A.K. and J.R. thank the Austrian and Slovak Academies of Sciences for financing the exchange of scientists. A.H. acknowledges the support of the Austrian Fond zu Förderung der wissenschaftlichen Forschung (No. 13308). We would like to thank H. Schleicher for the programs made for the IDL KIS LIB.
Copyright ESO 2002