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

(20) |

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.