[continues from 4.1.1]

4.1.2 Vignetting correction

The off-axis optics area (at an angle theta from the optical axis) is obtained by multiplying the on-axis area for a vignetting correction which has also been originally calibrated on the ground, and revised according to Crab observations and is interpolated as a function of energy from a set of coefficients.

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 :

    V(r,E) = (1 + V1(E) . r V2(E))-1
The conversion between r and theta (e.g. in arcmin) is given by the optics platescale. In the figure below the vignetting correction at different energies is reported as a function of theta.

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 :

Plot/compute vignetting correction V for MECS unit :
select ONE type of data representation, the relevant value and data format
V(theta) at given E keV
V(E) at given theta arcmin
V(x,y) image at E keV

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