The Instrumental Profile of the Utrecht Solar Atlas

Abstract

The Utrecht Atlas has been used a great deal by astronomers, and the solar equivalent widths derived from it are well entrenched in the literature. The method of oxygen-band convolutions shows that the second-order instrumental profile of the spectrograph used for the Atlas was much narrower than the authors of the Atlas supposed, having a half-width of about 40 mÅ at λ6000Å instead of 61 mÅ. The authors' error is largely in assuming that the observed half-widths of absorption lines result from quadratic addition of the intrinsic and instrumental half-widths. The failure of this assumption has important bearings on other astronomical measurements. Some effects of the newly derived instrumental profile upon absorption lines are calculated, and the asymmetry seen in absorption lines in the Utrecht Atlas is faithfully reproduced. For reasons associated with the instrumental profile, the equivalent widths of moderately strong lines will be under-measured by 2 to 10 per cent or more, depending upon the local complexity of the spectrum, while weak lines will be under-measured by about 15 per cent. Stellar chemical abundances derived ultimately from the use of Utrecht Atlas equivalent widths may correspondingly be too large by 15 per cent. An instrumental profile is for the first time deduced for the first-order spectra which constitute more than a third of the Atlas; although the resolving power is conspicuously poorer there than in the second order, the quantitative effects of the first-order profile upon the observed spectrum are less damaging than those of the second order as regards central intensities and equivalent widths.