The fraction of intercepted light equals for on-axis source positions and is tabulated in QuantumEf (see section 4.4.17). The function interpolates between the energy points tabulated.
For off-axis sources this parameterization is scaled with the
angle of incidence for on-axis sources according to
is dependent on energy due to the fact that the effective area of the mirror shells is a function of energy (smaller shells have higher effective area at higher energies).
is calculated from the dispersion component of the
off-axis position of the source in the following way: using the TELCOORD coordinate system, the position of the source in the
field-of-view is given by angles & . It follows then
where is the focal length of the respective mirror module, and is the distance between RGS focal camera (RFC) and Reflection Grating Array (RGA).