The conversion factor between the various magnetographs. a) Black: the MDI(1)-to-SPM conversion factor as a function of the MDI(1) magnetogram signal, |B_{los, MDI(1)}| at 0.97 < μ ≤ 1.00 (solid curves), 0.55 < μ ≤ 0.64 (dashed curves), and 0.20 < μ ≤ 0.55 (dotted curves). Red: the corresponding bivariate polynomial in |B_{los, MDI(1)}| and μ fit (Eq. (4)). The points below three times the magnetogram noise threshold (blue line) were excluded from the fit. Green: the global conversion factor (Eq. (5)). Grey: the conversion factor-versus-|B_{los, MDI(1)}| profile in each of the four quadrants of the solar disc. b) to g) The corresponding plots for the other instrument comparisons. See Sect. 3.2 for details.