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
and the angle of incidence on the gratings is given by
and The distance between and a virtual focus of the
The hardware related dispersion angle is calculated from the
ray from point to the detector coordinates of the event
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.