Coefficients are kept in files
`m{1,2,3}_vignetting.coeff`.

The coefficients give the vignetting correction as a function of the off-axis position
*r* in mm according to the following formula :

The conversion betweenV(r,E) = (1 + V_{1}(E) . r^{V2(E)})^{-1}

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Fig. 4.1.2-I : examples of MECS vignetting correction as a function of off-axis angle for the three MECS units, and for three energies (1.5, 3.2 and 8.1 keV)

Additional representations (as separately numbered figures) can be obtained via the following form :