Calibration Access and Data Handbook

Procedure

The fraction of intercepted light equals $\eta$ 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 $\alpha_0$ according to

$$\eta' = \eta\,\frac{\alpha}{\alpha_0}\ .$$

$\eta$ 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).

$\alpha$ 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 $\phi$ & $\theta$. It follows then

$$\alpha = \alpha_0 - \frac{F}{L}\ \theta\ \cos\phi \ ,$$ (28)

where $F$ is the focal length of the respective mirror module, and $L$ is the distance between RGS focal camera (RFC) and Reflection Grating Array (RGA).