[continues from 4.1.2]
A further correction is necessary to reduce the measurements done at Panter at a finite
source distance of 130 m (divergent beam) to the fligth condition of sources at infinity
(parallel beam). The correcting coefficients have been derived by comparison of ground
measurement with results of a ray tracing code. As such they are the same for all 3 MECS
The correcting factor is interpolated as a function of energy and off-axis angle theta from
a set of coefficients kept in file
The coefficients correspond to a cubic polynomial in theta.
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Fig. 4.1.3-I : examples of the Panter-to-infinity correction as a function of off-axis angle for three energies (1.5, 3.2 and 8.1 keV)
Additional representations (as separately numbered figures) can be obtained via the following form. inclusive of the overall vignetted area (compound calibration parameter, given by
the product of the various corrections) :
The focal length of the mirrors is 1850 mm, which corresponds to a platescale (value read
from provisional calibration file mir.dat) of
arcsec per mm in the focal plane.
The correspondence between mm and image pixels depends on the choice of the
linearized pixel size during the linearization procedure.
The unlinearized pixel sizes close to the detector centre are the average of
the 3rd and 4th linearization coefficient in file
- for M1 .17141 mm (average of .17600 on x and .16682 on y)
- for M2 .16326 mm (average of .16354 on x and .16298 on y)
- for M3 .17509 mm (average of .17561 on x and .17458 on y)