Top panel: galactic extinction in the LSST g filter (in magnitude) as a function of the value of pD along the line of sight for an elliptical galaxy with no intrinsic dust () at z = 0.1 (in red). A linear approximation A_g = apD where a the slope between (0,0) and (0.3,A_g(0.3)) is shown in black. The first cumulative orders of the mathematical decomposition of this function in Taylor series are shown by the dotted lines (see Eq. (A.2); , etc.). The grey vertical lines indicate the range of Galactic extinction at fixed pD for a library of 2604 extragalactic source SED (see Sect. 3.5). Bottom: differences between the real value of Galactic extinction computed from Eqs. (1) and (2) and the linear and Taylor series approximations. The scatter of extinction values for a range of SEDs is also reproduced in grey.