Average centre-to-limb variation in several strong lines from USUN mosaics (diamonds). For the He i 584 Å and O v 630 Å, where SYNOP data are available, squares indicate SYNOP data during the 2007–2009 solar minimum, while circles denote data during the 1996–1997 minimum. The quadratic (solid lines) and power-law (dashed lines) fits to the USUN data are also shown. Grey points (μ < 0.1) indicate data not used in the actual fits.