Parameters characterising the centre-to-limb variation in the O v 630 Å line during the last two solar minima. Diamonds: USUN; squares: SYNOP (2007–2009), circles: SYNOP (1996–1997). The average values are indicated by horizontal lines: dotted (USUN), solid (SYNOP, 2007–2009), and dashed (SYNOP, 1996–1997). In the upper-left panel, the centre-disk intensity, I(1), from the power-law fit are in black, while results from quadratic fits are in grey. The correction factors f_cl, whose average values are listed also in Table 1, are shown in the right-hand panels; the value of centre-to-limb function at ρ = 0.85 (i.e. μ ~ 0.52), derived from the quadratic fit of Eq. (2), is shown in the lower-left panel.