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: http://xmm.vilspa.esa.es/calibration/documentation/CALHB/node405.html
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For this calculation the grating assembly is approximated by an
imaginary grating placed at the center of the grating assembly at
point .
The angle of incidence on the facets of the gratings
where is the blaze angle, which is available from CCF MiscData, item BLAZE_ANGLE, and
is the angle of
incidence measured from the grating surface for an on axis source.
is available from CCF MiscData, item INCIDENCE_ANGLE, or from CCF Lincoord, item G_ALPHA.
Since the RGA may be rotated around , the actual angle of
incidence for an on-axis source is
is stored in CCF Lincoord, item G_RY.
With an off-axis source at given angular pair in the
TELCOORD reference frame, the component along the RGS dispersion
direction is
and the angle of incidence on the gratings is given by
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and The distance between and a virtual focus of the
telescope
is
The hardware related dispersion angle is calculated from the
ray from point
to the detector coordinates of the event
, as
with the radius of the Rowland circle available from CCF MiscData item ROWLAND.
And finally the defocus value is defined as X-component of
the distance between
and the intersection of the ray
with the Rowland circle, and is calculated by
The first component is the distance between detector bin and mid-point
of the grating assembly along the TELCOORD X-axis; the second
item is the distance between point G and the intersection of the
dispersed ray with the Rowland circle; the third component is a
correction term for a shift of the Rowland circle from it nominal
geometry due to a shift of point G during the integrations.
is the focal length of the telescope, which is available from CCF
MiscData, item FOCAL_LENGTH, and
is a linear offset that is taken from the CCF
LINESPREADFUNC, table FIGURE. This is a convenient
quantity to vary the defocus, without a distortion of the wavelength
scale.