[continues from 4.1.4]

4.1.5 Misalignments

The misalignments between the spacecraft axes XYZ and the detector axes of each MECS unit xyz can be defined by three angles as follows :

[definition of misalignment angles]

Fig. 4.1.5-I : each panel shows the rotation from the thicker axis system to the thinner one. The first three panels give the individual rotations described above, while the last one gives the overall misalignments. The orientation and rotation relative magnitude corresponds to unit M3, but rotation angles are exaggerated by a factor 100

The relevant coefficients have been calibrated from raster scan observations of a given celestial sources at known pointings. For all practical applications there is no significant rotation (within a couple of degrees) around the Z axis, hence alpha has been fixed to 0 ° for unit M3 or 180 ° for unit M1 and M2 (to give the same orientation as the spacecraft axes), while angle beta corresponds to a shift of about 10 arcmin along the x axis for all detectors, and gamma is smaller (between half and 2.5 arcmin) with uncertainties of about a quarter of arcmin.

[MECS unit orientation]

Fig. 4.1.5-II : orientation of MECS units with respect to the spacecraft XY axes

The effect of this is that if the satellite is pointed with its Z axis on the target, the latter will be positioned behind the strongback. This is visible in the first light pointing. The pointing strategy takes account of this, and compensates, placing the source close to the MECS centre, as in the second light pointing.

The coefficients are stored in files m{1,2,3}.misalignment and are used by software for conversion between image and sky coordinates.

4.2 Plasma suppression grid transmission

The transmission of the plasma suppression grid, taug, is a hardcoded scalar (energy-independent) value of FILT1=0.921 .

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