Spectral-line profiles in daytime skylight

The 1.2 m telescope and coudé spectrograph at our Elginfield Observatory was used to investigate the suitability of daytime skylight as a reference spectrum for astrophysical work. We show that with sufficient care, skylight does give the flux (or disk-integrated) spectrum of the Sun, allowing direct comparison with other stars. The line profiles in skylight become shallower with increasing angular separation from the Sun up to ≈ 100°, and then for larger angles they deepen slightly again. The amplitude of this variation is typically 3%-4% of the depth of the line. If the profiles are normalized to their central depths, their shapes remain unaltered. This implies that astrophysical analyses based on the shapes of spectral lines or ratios of depths of spectral lines in skylight should not be seriously affected. However, equivalent widths are not conserved; they can be reduced by up to the same 3%-4% unless care is taken to observe the sky close to the Sun or unless the angular variation is measured, as described here, and corrections applied. The skylight variations can be explained as a combination of aerosol and Rayleigh-Brillouin scattering. There is no discernible dependence with altitude of the Sun, ruling out a significant ground-albedo effect.