We have verified that the line wavelength shifts measured in the FTS disc-centre spectrum and the FTS flux spectrum agree on an absolute scale and do not show any strong trend with wavelength, whereas the Liège Atlas exhibits an anomalous wavelength dependence with line shifts in excess of the gravitational redshift, thereby revealing calibration errors.
Accurate wavelengths have been measured for 1446 FeI lines, both from intensity and flux spectra, providing the largest database available for comparison with other solar-type stars. This line list may be useful in employing solar (daylight or lunar) spectra to perform wavelength calibrations in high-resolution spectroscopy, or to assess the quality of calibrations based on spectral lamps, which normally illuminate the detector through a different optical path from that of the observational target.
Finally, it has been shown how the wavelength shifts of lines formed at the top of the photosphere get close to the gravitational redshift.
Acknowledgements
We thank H. Neckel for the help in dealing with his solar atlas, F. Thèvenin and A.D. Wittmann for providing us with a copy of their digital solar line lists, and H.H.R. Kroll for installing and maintaining the KIS computer libraries at the IAC. We are grateful to J. Sánchez Almeida for interesting comments after careful reading of the manuscript. Valuable bibliographic information was pointed out to us by the referee. NSO/Kitt Peak FTS data used here were produced by NSF/NOAO.
This work was partially supported by the Spanish DGES under projects PB92-0434-C02-01 and PB95-1132-C02-01.